Correct `peer_handler::test_process_events_multithreaded`

Correct `peer_handler::test_process_events_multithreaded`

This test was added some time ago in
0c034e9a82e4339fb32af9da63832ac2a64abb0b, but never made any sense.
`PeerManager::process_events` will go around its loop as many times
is required to ensure we've always processed all events which were
pending prior to a `process_events` call, so having a test that
checks that we never go around more than twice is obviously broken.

And, indeed, in CI this tests fails with some regularity.

Instead, the test here is changed to ensure that we detectably go
around the loop again at least once.

Fixes #2385