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>

tests/unit/libstore/data/derivation/simple.json

@@ -0,0 +1,25 @@
+{
+  "args": [
+    "bar",
+    "baz"
+  ],
+  "builder": "foo",
+  "env": {
+    "BIG_BAD": "WOLF"
+  },
+  "inputDrvs": {
+    "/nix/store/c015dhfh5l0lp6wxyvdn7bmwhbbr6hr9-dep2.drv": {
+      "dynamicOutputs": {},
+      "outputs": [
+        "cat",
+        "dog"
+      ]
+    }
+  },
+  "inputSrcs": [
+    "/nix/store/c015dhfh5l0lp6wxyvdn7bmwhbbr6hr9-dep1"
+  ],
+  "name": "simple-derivation",
+  "outputs": {},
+  "system": "wasm-sel4"
+}