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/libutil/hash.cc

@@ -158,6 +158,10 @@ bool Hash::IsValidBase32(absl::string_view s) {
   return true;
 }
 
+std::string Hash::ToStorePathHash() const {
+  return compressHash(*this, kStorePathHashSize).to_string(Base32, false);
+}
+
 static std::string printHash32(const Hash& hash) {
   assert(hash.hashSize);
   size_t len = hash.base32Len();

third_party/nix/src/libutil/hash.hh

@@ -8,6 +8,9 @@
 
 namespace nix {
 
+// Size of the hashes rendered in store paths, in bytes
+constexpr unsigned int kStorePathHashSize = 20;
+
 MakeError(BadHash, Error);
 
 // TODO(grfn): Replace this with the hash type enum from the daemon proto so we
@@ -90,6 +93,9 @@ struct Hash {
   /* Returns whether the passed string contains entirely valid base32
      characters. */
   static bool IsValidBase32(absl::string_view s);
+
+  // Convert this Hash to the format expected in store paths
+  [[nodiscard]] std::string ToStorePathHash() const;
 };
 
 /* Print a hash in base-16 if it's MD5, or base-32 otherwise. */