Speed up test_driver CircleCI plan

[mcgibbon]

Purpose

This PR updates the test_driver CircleCI plan to cache its gt4py caches for faster execution, and reduces the simulation time of the caching comm example (which is run as part of CI).

Infrastructure changes:

• test_driver circleci plan now caches .gt_cache
• read/write examples for caching comm now only run two timesteps instead of one hour

Checklist

Before submitting this PR, please make sure:

• You have followed the coding standards guidelines established at Code Review Checklist.
• Docstrings and type hints are added to new and updated routines, as appropriate
• All relevant documentation has been updated or added (e.g. README, CONTRIBUTING docs)
• For each public change and fix in pace-util, HISTORY has been updated
• Unit tests are added or updated for non-stencil code changes

[elynnwu]

lgtm