testbench: make test more reliable

There was a race between tcpflood and rsyslog in imptpc_maxsessions.sh.
We now use the new -A tcpflood option to make the timing more
predictable, hopefully fixing test flakiness.

Note: if that does not help, we need to introduce a wait on the number
of error messages and maybe a delay before tcpflood termination. The
theory behind the latter is that rsyslog possibly does not fully
iniaitlize session which are quickly aborted before rsyslog receives
the related OS notification! We just record this info in case we
need it and are positive that this change will fix the situation.

tests/imptcp_maxsessions.sh

@@ -35,7 +35,8 @@ startup
 echo "INFO: RSYSLOG_OUT_LOG: $RSYSLOG_OUT_LOG"
 
 echo "About to run tcpflood"
-tcpflood -c$CONNECTIONS -m$NUMMESSAGES -r -d100 -P129
+tcpflood -c$CONNECTIONS -m$NUMMESSAGES -r -d100 -P129 -A
+echo "-------> NOTE: CLOSED REMOTELY messages are expected and OK! <-------"
 echo "done run tcpflood"
 shutdown_when_empty
 wait_shutdown