common/dump: do not override test cleanup trap

common/dump: do not override test cleanup trap

Instead, call _cleanup_dump explicitly from a private _cleanup.
Remove the generic cleanup bits (rm $tmp.*) from _cleanup_dump.

The only xfs/dump test that had anything other than rm $tmp.* in
_cleanup in xfs/287, but that was _scratch_unmount, which is not
needed anyway.

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