iotests: add 248: test resume mirror after auto pause on ENOSPC

Test that mirror job actually resume on resume command after being
automatically paused on ENOSPC error.

It's a follow-up test for 8d9648cb
    "blockjob: fix user pause in block_job_error_action"

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Tested-by: John Snow <jsnow@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>