Move tests to separate directories, and document

Today, with the tests inside a `tests` intermingled with the
corresponding library's source code, we have a few problems:

- We have to be careful that wildcards don't end up with tests being
  built as part of Nix proper, or test headers being installed as part
  of Nix proper.

- Tests in libraries but not executables is not right:

  - It means each executable runs the previous unit tests again, because
    it needs the libraries.

  - It doesn't work right on Windows, which doesn't want you to load a
    DLL just for the side global variable . It could be made to work
    with the dlopen equivalent, but that's gross!

This reorg solves these problems.

There is a remaining problem which is that sibbling headers (like
`hash.hh` the test header vs `hash.hh` the main `libnixutil` header) end
up shadowing each other. This PR doesn't solve that. That is left as
future work for a future PR.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

(cherry picked from commit 91b6833686)

tests/unit/libstore/common-protocol.cc

@@ -0,0 +1,187 @@
+#include <regex>
+
+#include <nlohmann/json.hpp>
+#include <gtest/gtest.h>
+
+#include "common-protocol.hh"
+#include "common-protocol-impl.hh"
+#include "build-result.hh"
+#include "tests/protocol.hh"
+#include "tests/characterization.hh"
+
+namespace nix {
+
+const char commonProtoDir[] = "common-protocol";
+
+class CommonProtoTest : public ProtoTest<CommonProto, commonProtoDir>
+{
+public:
+    /**
+     * Golden test for `T` reading
+     */
+    template<typename T>
+    void readProtoTest(PathView testStem, const T & expected)
+    {
+        CharacterizationTest::readTest(testStem, [&](const auto & encoded) {
+            T got = ({
+                StringSource from { encoded };
+                CommonProto::Serialise<T>::read(
+                    *store,
+                    CommonProto::ReadConn { .from = from });
+            });
+
+            ASSERT_EQ(got, expected);
+        });
+    }
+
+    /**
+     * Golden test for `T` write
+     */
+    template<typename T>
+    void writeProtoTest(PathView testStem, const T & decoded)
+    {
+        CharacterizationTest::writeTest(testStem, [&]() -> std::string {
+            StringSink to;
+            CommonProto::Serialise<T>::write(
+                *store,
+                CommonProto::WriteConn { .to = to },
+                decoded);
+            return to.s;
+        });
+    }
+};
+
+#define CHARACTERIZATION_TEST(NAME, STEM, VALUE) \
+    TEST_F(CommonProtoTest, NAME ## _read) { \
+        readProtoTest(STEM, VALUE); \
+    } \
+    TEST_F(CommonProtoTest, NAME ## _write) { \
+        writeProtoTest(STEM, VALUE); \
+    }
+
+CHARACTERIZATION_TEST(
+    string,
+    "string",
+    (std::tuple<std::string, std::string, std::string, std::string, std::string> {
+        "",
+        "hi",
+        "white rabbit",
+        "大白兔",
+        "oh no \0\0\0 what was that!",
+    }))
+
+CHARACTERIZATION_TEST(
+    storePath,
+    "store-path",
+    (std::tuple<StorePath, StorePath> {
+        StorePath { "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo" },
+        StorePath { "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo-bar" },
+    }))
+
+CHARACTERIZATION_TEST(
+    contentAddress,
+    "content-address",
+    (std::tuple<ContentAddress, ContentAddress, ContentAddress> {
+        ContentAddress {
+            .method = TextIngestionMethod {},
+            .hash = hashString(HashType::htSHA256, "Derive(...)"),
+        },
+        ContentAddress {
+            .method = FileIngestionMethod::Flat,
+            .hash = hashString(HashType::htSHA1, "blob blob..."),
+        },
+        ContentAddress {
+            .method = FileIngestionMethod::Recursive,
+            .hash = hashString(HashType::htSHA256, "(...)"),
+        },
+    }))
+
+CHARACTERIZATION_TEST(
+    drvOutput,
+    "drv-output",
+    (std::tuple<DrvOutput, DrvOutput> {
+        {
+            .drvHash = Hash::parseSRI("sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc="),
+            .outputName = "baz",
+        },
+        DrvOutput {
+            .drvHash = Hash::parseSRI("sha256-b4afnqKCO9oWXgYHb9DeQ2berSwOjS27rSd9TxXDc/U="),
+            .outputName = "quux",
+        },
+    }))
+
+CHARACTERIZATION_TEST(
+    realisation,
+    "realisation",
+    (std::tuple<Realisation, Realisation> {
+        Realisation {
+            .id = DrvOutput {
+                .drvHash = Hash::parseSRI("sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc="),
+                .outputName = "baz",
+            },
+            .outPath = StorePath { "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo" },
+            .signatures = { "asdf", "qwer" },
+        },
+        Realisation {
+            .id = {
+                .drvHash = Hash::parseSRI("sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc="),
+                .outputName = "baz",
+            },
+            .outPath = StorePath { "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo" },
+            .signatures = { "asdf", "qwer" },
+            .dependentRealisations = {
+                {
+                    DrvOutput {
+                        .drvHash = Hash::parseSRI("sha256-b4afnqKCO9oWXgYHb9DeQ2berSwOjS27rSd9TxXDc/U="),
+                        .outputName = "quux",
+                    },
+                    StorePath { "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo" },
+                },
+            },
+        },
+    }))
+
+CHARACTERIZATION_TEST(
+    vector,
+    "vector",
+    (std::tuple<std::vector<std::string>, std::vector<std::string>, std::vector<std::string>, std::vector<std::vector<std::string>>> {
+        { },
+        { "" },
+        { "", "foo", "bar" },
+        { {}, { "" }, { "", "1", "2" } },
+    }))
+
+CHARACTERIZATION_TEST(
+    set,
+    "set",
+    (std::tuple<std::set<std::string>, std::set<std::string>, std::set<std::string>, std::set<std::set<std::string>>> {
+        { },
+        { "" },
+        { "", "foo", "bar" },
+        { {}, { "" }, { "", "1", "2" } },
+    }))
+
+CHARACTERIZATION_TEST(
+    optionalStorePath,
+    "optional-store-path",
+    (std::tuple<std::optional<StorePath>, std::optional<StorePath>> {
+        std::nullopt,
+        std::optional {
+            StorePath { "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo-bar" },
+        },
+    }))
+
+CHARACTERIZATION_TEST(
+    optionalContentAddress,
+    "optional-content-address",
+    (std::tuple<std::optional<ContentAddress>, std::optional<ContentAddress>> {
+        std::nullopt,
+        std::optional {
+            ContentAddress {
+                .method = FileIngestionMethod::Flat,
+                .hash = hashString(HashType::htSHA1, "blob blob..."),
+            },
+        },
+    }))
+
+}