common: enable testing of realtime quota when supported

common: enable testing of realtime quota when supported

If the kernel advertises realtime quota support, test it.

However, this has a plot twist -- because rt quota only works if the xfs
is formatted with rtgroups, we have to mount a filesystem to see if
rtquota is actually supported.  Since it's time consuming to format and
mount the scratch filesystem, we'll assume that the test and scratch
fses have the same support.

This will cause problems if one sets SCRATCH_RTDEV but not TEST_RTDEV.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>