Add test coverage for output_file not None cases for gaia and/or utils.tap

This is a follow-up for PR #2311. We went ahead to merge the changes for the release, and as the missing test coverage predates the PR they should not hold up the merge and release. See discussion: https://github.com/astropy/astroquery/pull/2311#pullrequestreview-894038779

cc @mhsarmiento

Start by locating the existing tests and implementations for gaia and utils.tap, then review PR #2311 and its linked discussion for the intended output_file behavior. Add coverage for output_file values that are not None and run the relevant test suite; done means those cases are exercised without delaying the release changes.

Written by the indexing model from the issue text.