feat(tvix/store): add rstest-based PathInfoService tests

This introduces rstest-based tests. We also add fixtures for creating
some BlobService / DirectoryService out of thin air.
To test a PathInfoService, we don't really care too much about its
internal storage - ensuring they work is up to the castore tests.

Change-Id: Ia62af076ef9c9fbfcf8b020a781454ad299d972e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11272
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/Cargo.nix

@@ -12988,6 +12988,14 @@ rec {
           }
         ];
         devDependencies = [
+          {
+            name = "rstest";
+            packageId = "rstest";
+          }
+          {
+            name = "rstest_reuse";
+            packageId = "rstest_reuse";
+          }
           {
             name = "tempfile";
             packageId = "tempfile";