Skip tests that need baserom if it doesn't exist

[bates64]

The BASEROM_US_URL secret isn't passed to PRs from people who don't have collaborator access (i.e. right now, anyone but me). For example: https://github.com/striezel-stash/merlon/actions/runs/4927238627/jobs/8803936605

This behaviour is fine, but we shouldn't fail CI tests for people without collaborator access. Instead, we should have certain tests that need the baserom run conditionally on whether BASEROM_US_URL is defined or not.