Expand TestPledgeBatching to test to test message propagation.

[Stebalien]

We need to end-to-end test batch sector proving in a devnet to make sure we don't have any hidden issues anywhere in the pipeline (e.g., message propagation because we'll have larger messages).

This will help us catch bugs like #6338.

[BigLep]

2021-05-28 discussion: @magik6k says we can do this as an integration test. We can look at TestPledgeBatching for inspiration.

Need to test PubSub as well.

[BigLep]

2021-06-17 standup: base work off #6329

[arajasek]

#6537 does this and is passing, just needs to land into the broader integ test work. This can be closed.