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/downstream-placeholder.cc

@@ -0,0 +1,41 @@
+#include <gtest/gtest.h>
+
+#include "downstream-placeholder.hh"
+
+namespace nix {
+
+TEST(DownstreamPlaceholder, unknownCaOutput) {
+    /**
+     * We set these in tests rather than the regular globals so we don't have
+     * to worry about race conditions if the tests run concurrently.
+     */
+    ExperimentalFeatureSettings mockXpSettings;
+    mockXpSettings.set("experimental-features", "ca-derivations");
+
+    ASSERT_EQ(
+        DownstreamPlaceholder::unknownCaOutput(
+            StorePath { "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo.drv" },
+            "out",
+            mockXpSettings).render(),
+        "/0c6rn30q4frawknapgwq386zq358m8r6msvywcvc89n6m5p2dgbz");
+}
+
+TEST(DownstreamPlaceholder, unknownDerivation) {
+    /**
+     * Same reason as above
+     */
+    ExperimentalFeatureSettings mockXpSettings;
+    mockXpSettings.set("experimental-features", "dynamic-derivations ca-derivations");
+
+    ASSERT_EQ(
+        DownstreamPlaceholder::unknownDerivation(
+            DownstreamPlaceholder::unknownCaOutput(
+                StorePath { "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo.drv.drv" },
+                "out",
+                mockXpSettings),
+            "out",
+            mockXpSettings).render(),
+        "/0gn6agqxjyyalf0dpihgyf49xq5hqxgw100f0wydnj6yqrhqsb3w");
+}
+
+}