btrfs: cleanup tests btrfs/030 and btrfs/034

btrfs: cleanup tests btrfs/030 and btrfs/034

As recently suggested by Dave Chinner, make use of the new function
named _run_btrfs_util_prog() to run the btrfs util program, and stop
using run_check for running xfs_io - instead filter xfs_io's output
and add it to the golden output.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>