fstests: use btrfs check repair for repairing btrfs filesystems

fstests: use btrfs check repair for repairing btrfs filesystems

There are two repair functions: _repair_scratch_fs() and
_repair_test_fs(). As the names suggest, these functions are designed to
repair the filesystems SCRATCH_DEV and TEST_DEV, respectively. However,
these functions never called proper comamnd for the filesystem type btrfs.
This patch fixes it. Thx.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>