Consolidate RunPromptEvalTestsUnittest mocking

Refactors the mocking used for RunPromptEvalTestsUnittest in
//agents/testing/eval_prompts_unittest.py. Previously, each test
specified a list of mocks via annotations which were largely identical
between all tests.

With this CL, mocking is instead handled as part of shared test setup.
This means that tests automatically get everything mocked to reasonable
defaults and only need to explicitly change behavior that they care
about.

Change-Id: I6c9a2485fb21e549bc6fce9a5a2d330dc47ebd7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6966513
Commit-Queue: Struan Shrimpton <sshrimp@google.com>
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1518239}