Add remote tests for gaia module

[bsipocz]

Currently there are only local tests. It would be great to have a few to help to spot upstream changes.

[eerovaher]

#1641 added a few remote tests. Is that enough to close this issue or should there be more?

[bsipocz]

I suspect the two tests we currently have don't really cover the module well, so more would be indeed better.

[bsipocz]

I was looking into closing this with the recently enabled doctesting of the narrative docs, but currently, we're having only 32% of the code covered in astroquery/gaia/core.py with remote tests. So I suppose either more tests or more docs would be nice.

(The number if up at 66% with non-remote tests included, so the overall situation is not that bad, we just need more safety lines for catching upstream issues)