Add loadgen to deployment integration test

[mhofman]

refs: #3444

Description

This change leverages the loadgen work, and stacks it on top of #4133 to run a short loadgen cycle against the 2 node testnet started by the integration test.

Unlike #3107, the goal is not to generate perf data (even though some very basic deploy times might be useful), but to make sure changes to agoric-sdk do not break the loadgen which is a simple representative of dapps.

It should also help catch some non-determinism source (see #3444), but only if caused by different SwingSet executions. It would probably not catch restart related non-determinism.

Security Considerations

This should not have any impact on security, and is modeled against the dapp integration test which also checks out another Agoric owned repository.

Testing Considerations

In general SDK breaking changes should be accommodated in the loadgen in a backwards compatible manner, so that the loadgen can run against both old and new versions of the SDK. That usually means merging a loadgen fix first, then running the integration test in the SDK's PR. If necessary, an SDK PR can target a specific loadgen branch using the following PR notation: #loadgen-branch: main (replace main with the branch name).

[michaelfig]

The approach and current details look great!

If minor tweaks to pass CI are all you need, you can consider this to be my approval. Rerequest a review only if you make other changes.

[mhofman]

@michaelfig I have not materially changed the PR since the review, so I will merge in a few unless I get any objections (after removing the PR trigger)