[Ops] Fix hard-coded output checking

[delanni]

Summary

Fixing a bad integration test:

• logs
• issue - but apparently not reported
• buildkite build

The output snapshot checking had current hard-coded in it, that only works on main.

fix: make output snapshot care for legacy branches, and how they might be included in the output

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: cce457d

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream