test(3p/nix): Set up scaffolding & fix up tests for value-to-{json|xml}

Configures the CMake build to load & run the GoogleTest tests.

I (grfn) also updated this to get the tests running as part of the nix
derivation, which required defining our own manual configurePhase and
installCheckPhase, rather than depending on the one provided by stdenv.
Not doing this would cause cmake to attempt to *run* the tests as part
of the buildPhase, which wouldn't work because the dynamic libraries
hadn't been put into a place where the test executables knew where to
find them. We're not sure *why* this fixes it, and for some reason
fixing this also breaks the automatic behavior of nixpkgs of passing
-j$NIX_BUILD_CORES -l$NIX_BUILD_CORES to make, but that's eaasy enough
to fix manually in a preBuild

Paired-With: Griffin Smith <grfn@gws.fyi>
Change-Id: I79d61854a3ff47301cdce8a40c76820a97bdf901
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1240
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libstore/store-api.cc

@@ -446,7 +446,7 @@ PathSet Store::queryValidPaths(const PathSet& paths,
    responsibility of the caller to provide a closure. */
 std::string Store::makeValidityRegistration(const PathSet& paths,
                                             bool showDerivers, bool showHash) {
-  std::string s = s;
+  std::string s;
 
   for (auto& i : paths) {
     s += i + "\n";