Unmounts can be slow and break quorum-heartbeat-timeout

We observe that unmount in this test can consume up to 10sec of time
before proceeding to record heartbeat timeout elections by followers.

When this happens, elections and new leaders happen before unmount even
completes. This indicates that hearbeat packets from the unmount are
ceased immediately, but the unmount is taking longer doing other things.
The timeouts then trigger, possibly during the unmount.

The result is that with timeouts of 3 seconds, we're not actually
waiting for an election at all. It already happened 7 seconds ago. The
code here just "sees" that it happens a few hundred ms after it started
looking for it.

There's a few ways about this fix. We could record the actual timestamp
of the election, and compare it with the actual timestamp of the last
heartbeat packet. This would be conclusive, and could disregard any
complication from umount taking too long. But it also means adding
timestamping in various places, or having to rely on tcpdump with packet
processing.

We can't just record $start before unmount. We will still violate the
part of the test that checks that elections didn't happen too late.
Especially in the 3sec test case if unmount takes 10sec.

The simplest solution is to unmount in a bg thread, and circle around
later to `wait` for it to assure we can re-mount without ill effect.

Signed-off-by: Auke Kok <auke.kok@versity.com>

tests/tests/quorum-heartbeat-timeout.sh

@@ -62,7 +62,7 @@ test_timeout()
 	sleep 1
 
 	# tear down the current server/leader
-	t_force_umount $sv
+	t_force_umount $sv &
 
 	# see how long it takes for the next leader to start
 	start=$(time_ms)
@@ -73,6 +73,7 @@ test_timeout()
 	echo "to $to delay $delay" >> $T_TMP.delay
 
 	# restore the mount that we tore down
+	wait
 	t_mount $sv
 
 	# make sure the new leader delay was reasonable, allowing for some slack