iotests/030: Reduce run time by unthrottling job earlier

iotests/030: Reduce run time by unthrottling job earlier

test_overlapping_3() throttles its active commit job so it can be sure
the job is still busy when it checks that you can't start a conflicting
streaming job.

However, it only sets the commit job back to full speed when it is
ready, which takes a few seconds while it's throttled. We can already
reset the limit after having checked that block-stream returns an error
and save these seconds.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200513100025.33543-1-kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>