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/libstore/tests/derived-path.hh

@@ -1,39 +0,0 @@
-#pragma once
-///@file
-
-#include <rapidcheck/gen/Arbitrary.h>
-
-#include <derived-path.hh>
-
-#include "tests/path.hh"
-#include "tests/outputs-spec.hh"
-
-namespace rc {
-using namespace nix;
-
-template<>
-struct Arbitrary<SingleDerivedPath::Opaque> {
-    static Gen<SingleDerivedPath::Opaque> arbitrary();
-};
-
-template<>
-struct Arbitrary<SingleDerivedPath::Built> {
-    static Gen<SingleDerivedPath::Built> arbitrary();
-};
-
-template<>
-struct Arbitrary<SingleDerivedPath> {
-    static Gen<SingleDerivedPath> arbitrary();
-};
-
-template<>
-struct Arbitrary<DerivedPath::Built> {
-    static Gen<DerivedPath::Built> arbitrary();
-};
-
-template<>
-struct Arbitrary<DerivedPath> {
-    static Gen<DerivedPath> arbitrary();
-};
-
-}