tests: last: skip when last is not enabled

References: https://bugs.gentoo.org/489794
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
tests: libmount: skip tabfiles-py when pylibmount is disabled

The default configure disables python support, so this test fails by
default.  Add a simple check for module availability.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>