Run all unit tests for golden files

[parthea]

Currently for golden files we only have lint and prerelease_deps presubmit checks. See below:

goldens-lint - runs mypy and lint checks https://github.com/googleapis/gapic-generator-python/blob/8d6a035f2c91cab0bc9fad25b7c0af5c5bfa63af/.github/workflows/tests.yaml#L310-L328

goldens-prerelease - runs unit tests against pre-release dependencies https://github.com/googleapis/gapic-generator-python/blob/8d6a035f2c91cab0bc9fad25b7c0af5c5bfa63af/.github/workflows/tests.yaml#L329-L349

We could consider running unit tests as well, especially the unit tests which tests against the minimum version of dependencies: https://github.com/googleapis/gapic-generator-python/blob/main/gapic/templates/testing/constraints-3.7.txt.j2