test-fstab-generator: skip test impacted by /mnt symlink

test-fstab-generator: skip test impacted by /mnt symlink

On rpm-ostree distributions such as Fedora SilverBlue /mnt
(and other well known paths) will be a symlink to a location
under /var. The fstab generator emits correct output in this
case, however, the data does not match the expected output
stored in the source tree.

Rather than trying to adapt the test data, just skip this
single test scenario when we see /mnt is a symlink.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>