Increase test coverage

[paultiq]

While doing a little work testing 3.14/3.14t, added a few test cases to close a few uncovered branches:

The remaining gaps are for testing has_fnctl, which I'm not sure can ever occur on most OS's, since Windows already has a separate path for the Windows case.

Before

Name Stmts Miss Branch BrPart Cover Missing ---------------------------------------------------------------------- src/filelock/__init__.py 22 4 2 1 79% 41-44 src/filelock/_api.py 144 1 28 0 99% 237 src/filelock/_unix.py 38 2 4 0 95% 31-32 src/filelock/_util.py 19 0 4 1 96% 26->exit src/filelock/asyncio.py 109 5 22 5 92% 86-87, 215->218, 229->232, 241-243, 260->exit ---------------------------------------------------------------------- TOTAL 1038 12 82 7 98% 

After

Name Stmts Miss Branch BrPart Cover Missing ---------------------------------------------------------------------- src/filelock/__init__.py 22 4 2 1 79% 41-44 src/filelock/_unix.py 38 2 4 0 95% 31-32 ---------------------------------------------------------------------- TOTAL 1115 6 84 1 99% 

[gaborbernat]

@paultiq seems Windows is not happy 🤔

[paultiq]

Windows seems happier now