filter: match $TEST_* $SCRATCH_* in beginning of path string

filter: match $TEST_* $SCRATCH_* in beginning of path string

For example, if $TEST_DIR=/mnt, only replace instances of /mnt that
are in the beginning of a path string, e.g.:

"/mnt/mntA/mntB:/mnt/mntC" => "TEST_DIR/mntA/mntB:TEST_DIR/mntC"

With this change, there is no longer a need to check the case of
$TEST_DEV being a substring of $TEST_DIR, because that would mean
that $TEST_DIR is a prefix of $TEST_DIR and that is not possible.

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