Improve test coverage for fnmatch.filter and fnmatch.filterfalse - #161904

Increased test coverage for Lib/fnmatch.py from 89% to 91%, covering all remaining executable logic branches (partial branches reduced from 2 to 0; the remaining uncovered lines are strictly module-level imports and definitions).

Added tests:

• FilterTestCase.test_posixpath: tests fnmatch.filter() on the os.path is posixpath branch, verifying that non-matching items (case-sensitive on POSIX) are properly skipped in the loop for both str and bytes.
• FilterFalseTestCase.test_posixpath: tests fnmatch.filterfalse() on the os.path is posixpath branch (itertools.filterfalse) for both str and bytes.