Zstandard: fix frame boundaries

[Rogdham]

Summary

An alternative fix to #3697. Added test taken from that PR, with credit to @mbeijen in the commit metadata.

Fixes #3538.
Closes #3697.

The first commit of this PR is from #3725. I am marking it as draft until #3725 is merged.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Rogdham]

The test_zstd_truncated, because it truncated the start of the frame, was not testing something different compared to test_zstd_decoding_error, I fixed the test by truncating the end only.

Also moved one statement out of the loop for speedup. I added an extra test as well.