[tests] `compare_outputs`'s `AssertionError` can be massive

[zacharyburnett]

If the fits diff has a lot of lines, that is all pushed into the AssertionError and printed into the pytest log. This can make the test log thousands of lines long; is it possible to summarize this into a simple line count? https://github.com/spacetelescope/drizzlepac/blob/d345a84a487910c6827fa20f8fc79e09dab4c0a1/tests/resources.py#L212

https://github.com/spacetelescope/RegressionTests/actions/runs/10514121938/job/29131353758#step:29:37022

[stscijgbot-hstdp]

This issue is tracked on JIRA as HLA-1316.

[zacharyburnett]

useful in https://github.com/spacetelescope/RegressionTests/pull/122

[s-goldman]

closed by #1928