test/py: Use loop mounts if guestmount fails in filesystem tests

If guestmount isn't available on the system, filesystem test setup falls
back to using loop mounts to prepare its disk images. If guestmount is
available but fails to work, the tests are immediately skipped. Instead
of giving up on a guestmount failure, try using loop mounts as an
attempt to keep tests running.

Also stop checking if guestmount is in PATH, as trying to run a missing
guestmount can now follow the same failure codepath and fall back to
loop mounts anyway.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>