Use the new `locate_test_data()` helper to get access to test-data.
Guard the test with `have_test_data()` to skip it in case test-data
access is not available.
Use the `can_modify_immutable()` helper from the TestBase parent class
so we do not duplicate the code in multiple places. Similarly, make use
of the `have_rpm_ostree()` helper.
Extract the `suppress_oserror()` function from the ObjectManager and
make it available as utility for other code as well.
This also adds a bunch of tests that verify it works as expected.
Move the 'test_util_selinux.py' test into the module-unittest
subdirectory.
Drop the '__main__' hookup while at it. `python -m unittest --help`
explains how you can run individual tests.
