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>

mk/common-test.sh

@@ -1,7 +1,7 @@
 # Remove overall test dir (at most one of the two should match) and
 # remove file extension.
 test_name=$(echo -n "$test" | sed \
-    -e "s|^unit-test-data/||" \
+    -e "s|^tests/unit/[^/]*/data/||" \
     -e "s|^tests/functional/||" \
     -e "s|\.sh$||" \
     )