Fix etcd test failures

Running
```
make -C deps/rabbitmq_peer_discovery_etcd ct-system
```
on some macOS system causes test failures because the client cannot
connect to etcd:
```
test failed to connect [localhost:2379] by <Gun Down> {down,
                                                       {shutdown,
                                                        econnrefused}}
```

The etcd log file didn't show any error message.
However, the etcd log file showed that the etcd listener got started
after the test case tried to connect.

This commit fixes the test failure.

A better solution would be to use the HTTP API or the etcdctl CLI to
poll the listener status. However, simply waiting for 2 seconds is good
enough for this test suite.

deps/rabbitmq_peer_discovery_etcd/test/system_SUITE.erl

@@ -203,6 +203,8 @@ start_etcd(Config) ->
            "--initial-cluster-token", "test-token",
            "--log-level", "debug", "--log-outputs", "stdout"],
     EtcdPid = spawn(fun() -> rabbit_ct_helpers:exec(Cmd) end),
+    %% Wait for etcd to start its listeners.
+    timer:sleep(2000),
 
     EtcdEndpoint = rabbit_misc:format("~s:~b", [EtcdHost, EtcdClientPort]),
     rabbit_ct_helpers:set_config(