MAINT: use slow marker instead of skip for slow tests

Tests a couple of tests that are marked skipped as they are run for a long time, or would download a large chunk of data. Instead of skipping them we should use the `slow` pytest marker: https://github.com/astropy/pytest-astropy/pull/48

Also, maybe add a largedata marker or similar, as the `hugemem` may not really be applicable for us.

Start by reviewing the skipped tests described in the issue and the referenced pytest-astropy pull request about the slow marker. Locate the relevant pytest marker configuration and determine which skips represent slow or large-data tests; done means the applicable tests use an appropriate marker and the need for a separate largedata marker is resolved.

Written by the indexing model from the issue text.