fstests: btrfs/261 fix failure if /var/lib/btrfs isn't writable

fstests: btrfs/261 fix failure if /var/lib/btrfs isn't writable

We don't need scrub status; it is okay to ignore the warnings due to
the readonly /var/lib/btrfs if any. Redirect stderr to seqres.full.
We check the scrub return status.

    +WARNING: failed to open the progress status socket at /var/lib/btrfs/scrub.progress.42fad803-d505-48f4-a04d-612dbf8bd724: Read-only file system. Progress cannot be queried
    +WARNING: failed to write the progress status file: Read-only file system. Status recording disabled

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