xfstests: always use test option when checking large scratch device

xfstests: always use test option when checking large scratch device

Some tests call _check_scratch_device directly and when using a
large filesystem this needs to run with a -t option to avoid
consuming large amounts of memory. Make this happen in all cases
that the scratch device is checked.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>