iotests: add #310 to test bottom node in COR driver

iotests: add #310 to test bottom node in COR driver

The test case #310 is similar to #216 by Max Reitz. The difference is
that the test #310 involves a bottom node to the COR filter driver.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
  [vsementsov: detach backing to test reads from top, limit to qcow2]
Message-Id: <20201216061703.70908-7-vsementsov@virtuozzo.com>
[mreitz: Add "# group:" line]
Signed-off-by: Max Reitz <mreitz@redhat.com>