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