List of tests to add

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At the moment we only have last-frames tests. Although they are useful, it's not enough. We would need to have tests for cli flags, that check videos, etc. This issue will keep a list of tests that need to be added. Please make more suggestions for tests!

## Tests that need to be added
- [ ] We should have at least one test for each CLI flag.
- [ ] We should test that the cascading priorities in the config system work as expected. Ties in with #201
- [ ] We should create a test to certify a clean install. See #11.
- [x] Testing video content
- [x] Test that manim runs correctly on all environments/prompts/versions #336
- [ ] Regression test for #240
- [ ] Test z-order stuff #327
- [ ] Test to ensure `MovingCamera` and `ZoomedScene` let the camera move as they should. From #388 .
- [ ] Test the functionality of asking the user for which scenes to render #523
- [x] There are no tests for the -s flag or the -t flag
- [ ] Test for `add_fixed_in_frame_mobjects` which is used in 3D scenes #569, currently broken
- [x] Tests for SVG images
- [ ] @XorUnison Once video tests are implemented `reverse_direction` should get one
- [ ] @XorUnison Test for Brace
- [ ] @PhilippImhof Graphical unit test for `Text` and `MarkupText` using a fixed font (TTF)

No specific file, test module, or entry point is named. Review the unchecked test ideas and choose one narrowly scoped item, such as CLI flags, configuration priority, or a regression from an issue reference. Done means the selected behavior has a repeatable test and the relevant test suite passes.

Written by the indexing model from the issue text.