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)
(cherry picked from commit a61e42adb5)

tests/unit/libutil/closure.cc

@@ -0,0 +1,70 @@
+#include "closure.hh"
+#include <gtest/gtest.h>
+
+namespace nix {
+
+using namespace std;
+
+map<string, set<string>> testGraph = {
+    { "A", { "B", "C", "G" } },
+    { "B", { "A" } }, // Loops back to A
+    { "C", { "F" } }, // Indirect reference
+    { "D", { "A" } }, // Not reachable, but has backreferences
+    { "E", {} }, // Just not reachable
+    { "F", {} },
+    { "G", { "G" } }, // Self reference
+};
+
+TEST(closure, correctClosure) {
+    set<string> aClosure;
+    set<string> expectedClosure = {"A", "B", "C", "F", "G"};
+    computeClosure<string>(
+        {"A"},
+        aClosure,
+        [&](const string currentNode, function<void(promise<set<string>> &)> processEdges) {
+            promise<set<string>> promisedNodes;
+            promisedNodes.set_value(testGraph[currentNode]);
+            processEdges(promisedNodes);
+        }
+    );
+
+    ASSERT_EQ(aClosure, expectedClosure);
+}
+
+TEST(closure, properlyHandlesDirectExceptions) {
+    struct TestExn {};
+    set<string> aClosure;
+    EXPECT_THROW(
+        computeClosure<string>(
+            {"A"},
+            aClosure,
+            [&](const string currentNode, function<void(promise<set<string>> &)> processEdges) {
+                throw TestExn();
+            }
+        ),
+        TestExn
+    );
+}
+
+TEST(closure, properlyHandlesExceptionsInPromise) {
+    struct TestExn {};
+    set<string> aClosure;
+    EXPECT_THROW(
+        computeClosure<string>(
+            {"A"},
+            aClosure,
+            [&](const string currentNode, function<void(promise<set<string>> &)> processEdges) {
+                promise<set<string>> promise;
+                try {
+                    throw TestExn();
+                } catch (...) {
+                    promise.set_exception(std::current_exception());
+                }
+                processEdges(promise);
+            }
+        ),
+        TestExn
+    );
+}
+
+}