[4/?] Move promptfoo installation code 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 promptfoo installation-related code into a new
promptfoo_installation.py file. This does not provide any immediate
benefit other than making eval_prompts.py less monolithic, but this
will be necessary for future changes since the worker pool will depend
on a promptfoo installation.

Also drive-by updates the moved tests to share mock setup between tests
instead of using repeated mock.patch annotations.

Bug: 445459870
Change-Id: I92d57e552949d9b37b5bf22f077f5ad8b04d244d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6981296
Commit-Queue: Struan Shrimpton <sshrimp@google.com>
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Cr-Commit-Position: refs/heads/main@{#1520668}