Add persistence tests using "malicious" skops files

[BenjaminBossan]

We designed the skops persistence format with the intent of being safe against maliciously crafted skops files. Right now, however, there are not tests that explicitly check that loading these kinds of files. Those tests should be added. Of course, nothing actually malicious should happen, but they should serve as a proof of concept.

Ideally, these tests should also help to increase code coverage, as we currently have some uncovered lines that exist as a safeguard against malicious files (see discussion starting here).

[E-Aho]

Hey! I'll jump on this, starting to feel a lot better now and I think this would be a good warm up :)