Add a new optional pytest CLI argument `--unsupported-fs` allowing to
specify file-systems which should be treated as unsupported in the
platform where running tests. Any test cases dependent on such
file-system support will be sipped.
This will allow to run unit tests and selectively skipping test cases
for unsupported file-systems.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>