Refactors our use of `slogtest` to instantiate a "standard logger" across most of our tests. This standard logger incorporates https://github.com/coder/slog/pull/217 to also ignore database query canceled errors by default, which are a source of low-severity flakes.
Any test that has set non-default `slogtest.Options` is left alone. In particular, `coderdtest` defaults to ignoring all errors. We might consider revisiting that decision now that we have better tools to target the really common flaky Error logs on shutdown.
Fixes#13139
Using a bare channel to signal dependent goroutines means that we can only signal success, not failure, which leads to deadlock if we fail in a way that doesn't cause the whole `apiConnRoutineManager` to tear down routines.
Instead, we use a new object called a `checkpoint` that signals success or failure, so that dependent routines get unblocked if the routine they depend on fails.
