fstests: cleanup test btrfs/031

fstests: cleanup test btrfs/031

The test was using $SCRATCH_MNT as a mountpoint for $SCRATCH_DEV, which
is counter intuitive and not expected by the fstests framework - this
made the test fail after commit 27d077ec0bda (common: use mount/umount
helpers everywhere). So rewrite the test to use the scratch device for
all data and use a test specific directory inside $TEST_DIR to use as a
mount point for a cross mount of $SCRATCH_DEV.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>