fstests: fix test and scratch filters for overlapping DEV/MNT paths

fstests: fix test and scratch filters for overlapping DEV/MNT paths

When configuring overlay base fs, TEST_DEV/DIR and SCRATCH_DEV/MNT
are derived from the base fs mount points, where *_DEV are the
path of the base fs mount point and TEST_DIR/SCRATCH_MNT are
a directory under the base fs mount point.

This means that the overlay DEV paths are prefixes of the overlay
mount points.
Fix the test and sctach filters to check if TEST_DEV/SCRATCH_DEV is
a substring of TEST_DIR/SCRATCH_MNT and try and match the longer
string first.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>