Add sleep to give enough time for server to be up

[Naireen]

From local tests, it looks like even though we publish 10 messages, the server isn't up yet, so we miss the first couple of messages which is why the assertions fail. What isn't clear to me is why that has transient failures, but the regular read test doesn't

History: https://develocity.apache.org/scans/tests?search.relativeStartTime=P28D&search.timeZoneId=America%2FLos_Angeles&tests.container=org.apache.beam.sdk.io.mqtt.MqttIOTest

This is flakey enough that you can see this running locally, but adding in a sleep got rid of it. Ideaaly there would be a nicer method of checking if the server was up and ready to accept messages, but couldn't figure out a nice quick easy way to do that.

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