The test code prior to r311893 loaded geom_gate at test start if necessary and
skipped the tests if it couldn't be loaded.
The ATF-ifcation of this test done in r311893 unfortunately dropped this
functionality.
This change restores the geom_gate module load and skips the test(s) if unavailable
in an ATF-like way.
MFC after: 1 month
PR: 220164
Reported by: gjb