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/libstore/data/path-info/impure.json

@@ -0,0 +1,16 @@
+{
+  "ca": "fixed:r:sha256:1lr187v6dck1rjh2j6svpikcfz53wyl3qrlcbb405zlh13x0khhh",
+  "deriver": "/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-bar.drv",
+  "narHash": "sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc=",
+  "narSize": 34878,
+  "references": [
+    "/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-bar",
+    "/nix/store/n5wkd9frr45pa74if5gpz9j7mifg27fh-foo"
+  ],
+  "registrationTime": 23423,
+  "signatures": [
+    "asdf",
+    "qwer"
+  ],
+  "ultimate": true
+}

tests/unit/libstore/data/path-info/pure.json

@@ -0,0 +1,9 @@
+{
+  "ca": "fixed:r:sha256:1lr187v6dck1rjh2j6svpikcfz53wyl3qrlcbb405zlh13x0khhh",
+  "narHash": "sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc=",
+  "narSize": 34878,
+  "references": [
+    "/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-bar",
+    "/nix/store/n5wkd9frr45pa74if5gpz9j7mifg27fh-foo"
+  ]
+}