btest/broker/publish-errors: Avoid exit(0)

Calling exit() doesn't properly shutdown the manager instances and TSAN reports a
leaked thread. Just avoid this for now by using terminate() instead.

testing/btest/Baseline.zam/broker.publish-errors/send..stderr

@@ -1,2 +1,3 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
 error in ../send.zeek, line 18: expected Broker::Event, got Cluster::Event (Broker::publish(/test/topic, <internal>::evt.18))
+received termination signal

testing/btest/Baseline/broker.publish-errors/send..stderr

@@ -1,2 +1,3 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
 error in ../send.zeek, line 18: expected Broker::Event, got Cluster::Event (Broker::publish(/test/topic, evt))
+received termination signal

testing/btest/broker/publish-errors.zeek

@@ -31,7 +31,7 @@ event Broker::peer_added(endpoint: Broker::EndpointInfo, msg: string)
 	local evt = Cluster::make_event(my_event, 42);
 	local r = Broker::publish("/test/topic", evt);
 	assert ! r;  # Supposed to fail.
-	exit(0);
+	terminate();
 	}
 # @TEST-END-FILE