PR summary

This PR fixes a bug with #27882, which intended to delete all images that have passed tests before uploading the artifact. However, due to the specification of -type f in the find call, it inadvertently skips over images where the expected image is a symbolic link rather than an actual file. All of the baseline images in the repository are symbolically linked in the results folder (#10222), so #27882 actually deleted only those images where the expected image was dynamically generated.

This PR also extends the cleanup to GIF files (of which there is exactly one test).

I've tested this modification in a different PR, so I know that it successfully reduces the size of the uploaded artifact by ~50 MB. A few images (total of 370 kB) manage to still escape the cleanup, but I think that's small enough to put up with:

• empty.eps and empty_eps.png from test_backend_ps.py, which seems to be because the comparison image is empty-expected.pdf (PDF, not EPS), so the cleanup isn't smart enough to clean them up
• Many of the images from test_backend_pgf.py, which use a different approach for image comparisons

PR checklist

• [N/A] "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines