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/libutil/compression.cc

@@ -0,0 +1,96 @@
+#include "compression.hh"
+#include <gtest/gtest.h>
+
+namespace nix {
+
+    /* ----------------------------------------------------------------------------
+     * compress / decompress
+     * --------------------------------------------------------------------------*/
+
+    TEST(compress, compressWithUnknownMethod) {
+        ASSERT_THROW(compress("invalid-method", "something-to-compress"), UnknownCompressionMethod);
+    }
+
+    TEST(compress, noneMethodDoesNothingToTheInput) {
+        auto o = compress("none", "this-is-a-test");
+
+        ASSERT_EQ(o, "this-is-a-test");
+    }
+
+    TEST(decompress, decompressNoneCompressed) {
+        auto method = "none";
+        auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        auto o = decompress(method, str);
+
+        ASSERT_EQ(o, str);
+    }
+
+    TEST(decompress, decompressEmptyCompressed) {
+        // Empty-method decompression used e.g. by S3 store
+        // (Content-Encoding == "").
+        auto method = "";
+        auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        auto o = decompress(method, str);
+
+        ASSERT_EQ(o, str);
+    }
+
+    TEST(decompress, decompressXzCompressed) {
+        auto method = "xz";
+        auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        auto o = decompress(method, compress(method, str));
+
+        ASSERT_EQ(o, str);
+    }
+
+    TEST(decompress, decompressBzip2Compressed) {
+        auto method = "bzip2";
+        auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        auto o = decompress(method, compress(method, str));
+
+        ASSERT_EQ(o, str);
+    }
+
+    TEST(decompress, decompressBrCompressed) {
+        auto method = "br";
+        auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        auto o = decompress(method, compress(method, str));
+
+        ASSERT_EQ(o, str);
+    }
+
+    TEST(decompress, decompressInvalidInputThrowsCompressionError) {
+        auto method = "bzip2";
+        auto str = "this is a string that does not qualify as valid bzip2 data";
+
+        ASSERT_THROW(decompress(method, str), CompressionError);
+    }
+
+    /* ----------------------------------------------------------------------------
+     * compression sinks
+     * --------------------------------------------------------------------------*/
+
+    TEST(makeCompressionSink, noneSinkDoesNothingToInput) {
+        StringSink strSink;
+        auto inputString = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        auto sink = makeCompressionSink("none", strSink);
+        (*sink)(inputString);
+        sink->finish();
+
+        ASSERT_STREQ(strSink.s.c_str(), inputString);
+    }
+
+    TEST(makeCompressionSink, compressAndDecompress) {
+        StringSink strSink;
+        auto inputString = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        auto decompressionSink = makeDecompressionSink("bzip2", strSink);
+        auto sink = makeCompressionSink("bzip2", *decompressionSink);
+
+        (*sink)(inputString);
+        sink->finish();
+        decompressionSink->finish();
+
+        ASSERT_STREQ(strSink.s.c_str(), inputString);
+    }
+
+}