test(tvix): Cover scanForReferences in a test

Aded a few test cases covering the scanForReferences function, which had
been accidentally broken in 976a36c (which is now partially-reverted).
As part of this, since the test needed to generate hashes for store
paths, the logic in MakeStorePath to compress a sha256 hash down to 20
bytes and convert it to base32 has been extracted to a member function
on the Hash class.

Fixes: #34
Change-Id: Ie2d914688a80f42d0234d351a7cc0714fd15709e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1698
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>

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

@@ -323,9 +323,8 @@ Path Store::makeStorePath(const std::string& type, const Hash& hash,
 
   checkStoreName(name);
 
-  return storeDir + "/" +
-         compressHash(hashString(htSHA256, s), 20).to_string(Base32, false) +
-         "-" + name;
+  return absl::StrCat(storeDir, "/", hashString(htSHA256, s).ToStorePathHash(),
+                      "-", name);
 }
 
 Path Store::makeOutputPath(const std::string& id, const Hash& hash,