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)

src/libexpr/tests/flakeref.cc

@@ -1,22 +0,0 @@
-#include <gtest/gtest.h>
-
-#include "flake/flakeref.hh"
-
-namespace nix {
-
-/* ----------- tests for flake/flakeref.hh --------------------------------------------------*/
-
-    /* ----------------------------------------------------------------------------
-     * to_string
-     * --------------------------------------------------------------------------*/
-
-    TEST(to_string, doesntReencodeUrl) {
-        auto s = "http://localhost:8181/test/+3d.tar.gz";
-        auto flakeref = parseFlakeRef(s);
-        auto parsed = flakeref.to_string();
-        auto expected = "http://localhost:8181/test/%2B3d.tar.gz";
-
-        ASSERT_EQ(parsed, expected);
-    }
-
-}