Add slit overlay tests

[javerbukh]

Description

Relies on #768 being merged.

This pull request is to add tests for the slit overlay plugin. The tests currently do not work (which is why this in draft form), but I believe the source is the image fixture in the conftest.py file, since it has trouble parsing the header values into WCS.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• [ ] Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• [ ] Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• [ ] Do the proposed changes follow the STScI Style Guides?
• [ ] Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• [ ] Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• [ ] Did the CI pass? If not, are the failures related?
• [ ] Is a change log needed? If yes, is it added to CHANGES.rst?
• [ ] Is a milestone set? Milestone is only currently required for PRs related to Imviz MVP.
• [ ] After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?