Add test for case where parser overwrite annotations

[polm]

Description

As pointed out in #7716, the parser can overwrite is_sent_start annotations if not all tokens in a doc have them. The exact circumstances for this are unclear, but specifically it can turn False annotations into True.

This just adds a failing test for that case. The bad news is the test is flaky and succeeds occasionally.

It might make sense to leave this PR around until a fix can be added to it, or to change the test to a non-strict xfail instead.

Types of change

Adds a test.

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[svlandeg]

Thanks!

I would move this test to tests/regression/test_issue7716.py. Anyone looking to fix the original issue should be writing the test at that spot, and should thus find this. And ofcourse the original issue is now linked to this PR so the trail is visible. But in general if there is an associated issue, we prefer having it as part of the regression test suite.