MAINT: Speed up slow tests

A number of remote tests are markes as slow, while https://github.com/astropy/astroquery/issues/2420 addresses changing how they are marked, the preferable solution would be to find smaller datasets so that the tests to not need to be marked slow.

Start by locating the remote tests currently marked as slow and identify the datasets each test uses. Review issue #2420 for the related marking change, then replace oversized datasets with smaller ones where possible and verify that the tests no longer need the slow designation.

Written by the indexing model from the issue text.