iotests/281: Test lingering timers

iotests/281: Test lingering timers

Prior to "block/nbd: Delete reconnect delay timer when done" and
"block/nbd: Delete open timer when done", both of those timers would
remain scheduled even after successfully (re-)connecting to the server,
and they would not even be deleted when the BDS is deleted.

This test constructs exactly this situation:
(1) Configure an @open-timeout, so the open timer is armed, and
(2) Configure a @reconnect-delay and trigger a reconnect situation
    (which succeeds immediately), so the reconnect delay timer is armed.
Then we immediately delete the BDS, and sleep for longer than the
@open-timeout and @reconnect-delay.  Prior to said patches, this caused
one (or both) of the timer CBs to access already-freed data.

Accessing freed data may or may not crash, so this test can produce
false successes, but I do not know how to show the problem in a better
or more reliable way.  If you run this test on "block/nbd: Assert there
are no timers when closed" and without the fix patches mentioned above,
you should reliably see an assertion failure.
(But all other tests that use the reconnect delay timer (264 and 277)
will fail in that configuration, too; as will nbd-reconnect-on-open,
which uses the open timer.)

Remove this test from the quick group because of the two second sleep
this patch introduces.

(I decided to put this test case into 281, because the main bug this
series addresses is in the interaction of the NBD block driver and I/O
threads, which is precisely the scope of 281.  The test case for that
other bug will also be put into the test class added here.

Also, excuse the test class's name, I couldn't come up with anything
better.  The "yield" part will make sense two patches from now.)

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>