perf(vulnfeeds): make vulnfeeds tests go faster

[jess-lowe]

1. TestReposFromReferences (in vulnfeeds/conversion):

   • Issue: Was taking ~25 seconds. One subtest involving a cgit repository was taking over 20 seconds, likely due to network timeouts or slow responses during go-git operations or canonical link validation.
   • Fix: Modified the test to use the VCR client for git.FindCanonicalLink calls (which perform HTTP HEAD requests). This allows recording and replaying these requests.
   • Result: Test time reduced to ~3 seconds.

2. cve5 Package Tests (vulnfeeds/conversion/cve5):

   • Issue: TestCVE5Snapshot and TestExtractVersions were doing live network calls during version extraction and Git version-to-commit conversion.
   • Fix: Integrated testutils.SetupGitVCR(t) into these tests to record and replay Git remote interactions.
   • Result: While initial recording still takes time, playback significantly reduces execution time. TestExtractVersions reduced from ~8s to ~3.8s (played back). TestCVE5Snapshot took around 10-12s, processing multiple files sequentially.

[another-rex]

Slightly hesitant on encoding git responses, but let's try it and see.

Can you extend/explain the final line though?

Result: While initial recording still takes time, playback significantly reduces execution time. TestExtractVersions reduced from ~8s to ~3.8s (played back). TestCVE5Snapshot took around 10-12s, processing multiple files sequentially.