btest/cluster/zeromq: Add tests for overload behavior

The overload-drop.zeek and overload-no-drop.zeek tests have proxy,
worker-1 and worker-2 publish to the manager topic. For the drop
case, we verify that both, the senders, but also the manager drops
events. For the no-drop test, the HWMs are set such that all events
are buffered.

The overload-worker-proxy-topic*.zeek tests are similar, but instead
of publishing to the manager topic, proxy, worker-1 and worker-2 publish
to the proxy and worker topics to overload each other. This had
previously resulted in lockups and these tests verify that this doesn't
happen anymore.

testing/btest/Baseline/cluster.zeromq.overload-worker-proxy-topic-drop/manager.out

@@ -0,0 +1,13 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+nodes_up, 1
+nodes_up, 2
+nodes_up, 3
+nodes_done, 1
+nodes_done, 2
+nodes_done, 3
+sending finish
+nodes_down, 1
+nodes_down, 2
+nodes_down, 3
+had xpub_drops?, F
+had onloop_drops?, F

testing/btest/Baseline/cluster.zeromq.overload-worker-proxy-topic-drop/proxy.out

@@ -0,0 +1,5 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+had xpub_drops?, T
+had onloop_drops?, T
+node worker-1 dropped=T
+node worker-2 dropped=T

testing/btest/Baseline/cluster.zeromq.overload-worker-proxy-topic-drop/worker-1.out

@@ -0,0 +1,5 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+had xpub_drops?, T
+had onloop_drops?, T
+node proxy dropped=T
+node worker-2 dropped=T

testing/btest/Baseline/cluster.zeromq.overload-worker-proxy-topic-drop/worker-2.out

@@ -0,0 +1,5 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+had xpub_drops?, T
+had onloop_drops?, T
+node proxy dropped=T
+node worker-1 dropped=T