iotests: Adjust which migration tests are quick

A quick run of './check -qcow2 -g migration' shows that test 169 is
NOT quick, but meanwhile several other tests ARE quick.  Let's adjust
the test designations accordingly.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200727195117.132151-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>