_require_prjquota: Disable tests only when using realtime fs

_require_prjquota: Disable tests only when using realtime fs

$USE_EXTERNAL needs to be set when using external log devices. In such a
setup, tests which have "_require_prjquota
$SCRATCH_DEV" (e.g. generic/383)  incorrectly end up being marked as
"not run" since the test "[ "$USE_EXTERNAL" = yes -a ! -z "$_dev" ]"
evaluates to true.

This commit fixes the bug by marking the test as "not run" only when
$USE_EXTERNAL is set and one of $TEST_RTDEV or $SCRATCH_RTDEV is set.

Signed-off-by: Chandan Rajendra <chandan@linux.ibm.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>