test: add checkskip hook

Although there is the skip() macro, this doesn't actually do anything to skip
the test. I looked into adding a test_skip() function in the zdtm lib, but
there didn't seem an easy race-free way to propagate an exit code up to
run_test in zdtm.sh, so we add the checkskip hook for use in the apparmor test
(e.g. when apparmor isn't avialable in the kernel, or is disabled).

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>