iotests/151: Test that active mirror progresses

iotests/151: Test that active mirror progresses

Before this series, a mirror job in write-blocking mode would pause
issuing background requests while active requests are in flight.  Thus,
if the source is constantly in use by active requests, no actual
progress can be made.

This series should have fixed that, making the mirror job issue
background requests even while active requests are in flight.

Have a new test case in 151 verify this.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20221109165452.67927-5-hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>