Task-local logger is set with nworkers == 0, but not with nworkers > 0

[Drvi]

This has led to some confusion in the past, when people set a global logger and sometimes the change took effect (nworkers > 0) and sometimes it didn't (nworkers==0, we have a task-local logger which takes precedence).

We’ve been using this pattern, when the coordinator process uses with_logger(current_logger()) do ... to get around the world age issues we’ve been seeing (e.g. here). This isn't strictly a noop as it does set the task-local logger, and we only do this for the nworkers==0 case.

Ideally, there wouldn't be a difference between the two cases -- we could remove the task local logger when running tests locally. We should be careful not to violate the following while doing so:

1. worker_init_expr should (IMO) be able to set the global / task local logger, so our change would need to happen before worker_init_expr (but we seem to silently ignore worker_init_expr with nworkers==0 which doesn't feel right)
2. The user should be able to control the internal logging of ReTestItems which happens after worker_init_expr

So to fully address this issue, I'd propose to
a) Respect worker_init_expr when nworkers==0 (this is somewhat orthogonal, but the current state doesn't seem right)
b) Capture the current logger before worker_init_expr and remove the current logger while test items are evaluated
c) Wrap all internal logs in ReTestItems with the original logger from b)