gh-115911: Ignore PermissionError during import from cwd (#116131)

Ignore PermissionError when checking cwd during import

On macOS `getcwd(3)` can return EACCES if a path component isn't readable,
resulting in PermissionError. `PathFinder.find_spec()` now catches these and
ignores them - the same treatment as a missing/deleted cwd.

Introduces `test.support.os_helper.save_mode(path, ...)`, a context manager
that restores the mode of a path on exit.

This is allows finer control of exception handling and robust environment
restoration across platforms in `FinderTests.test_permission_error_cwd()`.

Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>
Co-authored-by: Brett Cannon <brett@python.org>