TST: Un-ignore currently ignored test warnings

I think this work would encompass multiple PRs. Right now, some warnings are in the blanket ignore like because they happen too often across the test suite or require time to track down. If possible, we should remove them one by one from the ignored list and handle them properly.

For example, when the code emits resource warning, either the test needs to gracefully close the file handler on exit (either pass or fail) or the code being tested is leaving a dangling file pointer (harder to fix).

https://github.com/astropy/specutils/blob/9a5bf9368586b420549c70e8279f31e93e567ad9/setup.cfg#L56

Start by inspecting the blanket warning ignore list around setup.cfg line 56 and identify one ignored warning category. Run the test suite to reproduce it, then trace whether the warning comes from a test handler or the code under test. Done means addressing that warning without relying on the blanket ignore; the issue suggests this may require separate PRs.

Written by the indexing model from the issue text.