fstests: filter test and scratch together safely

fstests: filter test and scratch together safely

Several tests uses both _filter_test_dir and _filter_scratch
concatenated by pipe to filter $TEST_DIR and $SCRATCH_MNT. However,
this would fail if the shorter string is a substring of the other
(like "/mnt" and "/mnt2").

This patch introduces new common filter function to safely call both
_filter_test_dir and _filter_scratch, and update tests and functions
to use this new function.

I checked this with btrfs/029, generic/409,410,411, and
generic/381,383, xfs/106,108 (which calls _filter_quota). Thanks
Eryu for advice.

[eguan: folded 2nd patch into 1st patch and update commit log a bit]

Signed-off-by: Tomohiro Misono <misono.tomohiro@jp.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>