add test that will fail on get_frame(clip.duration) bug

[blooop]

I was going to make an issue, but it was actually easier to add a test case that would fail on the bug I found.

I found that if I pass the duration of the clip to .get_frame() it does not return the last frame, but rather the first frame. even passing get_frame(clip.duration-1/clip.fps) does not return the last frame. Only get_frame.duration(clip.duration- 2/clip.fps) will return a frame near the end of the video (I have not confirmed if its actually the last frame).

I have confirmed the test fails locally. The errors are not exactly what I was expecting (I.e I expect the assertion to fail because the requesting get_frame(0) and get_frame(duration) return the same value) but instead I get indexing errors.

Maybe the reason for this will be clearer to someone who understands the codebase better, but I hope this test demonstrates the class of error.

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it

This is the context in which I found and fixed the bug for my usecase:
https://github.com/blooop/bencher/pull/551/files