Add Active -> Offline -> Faulted tests

[mkeeter]

This is (hopefully) the final prep PR before addressing #1252.

There was an untested path of Active -(timeout)-> Offline -(too many jobs)-> Faulted; this PR adds tests for both the job and byte-based fault conditions. In addition, it confirms that live-repair works after all fault-inducing tests, moving that logic to a new async fn run_live_repair(mut harness: TestHarness).

Making these tests pass required fixing a (probably innocuous) race condition:

• Downstairs is marked as offline, begins to reconnect (with a 10s pause)
• Too many jobs accumulate, and we try to stop the downstairs (to restart it again)
• ClientIoTask::run_inner ignores the stop message during the initial 10s pause
• After the pause completes, the client IO task connects then immediately disconnects, because it finally looks at the stop message

I think this was only an issue in unit tests, where we manage reconnections by hand; in the real system, the second reconnection should work fine. Anyhow, I fixed it by adding the stop condition to the initial client sleep and connection calls, so that it can interrupt the client at any point.

[leftwo]

I thought I had tested this path, but I believe I did so before I then
later changed the timeout before reconnect to 10 seconds!