[3/?] Move WorkDir to new file

This is part of a chain of CLs to add support for multiple parallel
workers to //agents/testing/eval_prompts.py.

This CL moves the WorkDir class into a new workers.py file. This does
not currently provide any real benefit other than making eval_prompts.py
less monolithic, but additional worker-related code will be added to
this file in follow-up CLs.

Also performs two drive-by improvements:
1. Makes the result thread daemonic so that it does not block the
   Python process from exiting.
2. Updates the moved tests for WorkDir to share mock setup between tests
   instead of using repeated mock.patch annotations.

Bug: 445459870
Change-Id: I37562b2e31577972ec61d8e24dd6e2309aa773ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6981947
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Struan Shrimpton <sshrimp@google.com>
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Cr-Commit-Position: refs/heads/main@{#1520664}