Remove "fast" versions of setup and test tasks

[aduth]

🛠 Summary of changes

Removes "fast" alternatives to make test and make setup, i.e. make fast_test and make fast_setup.

As a substitute, added a new paragraph to README.md encouraging running tests for individual files and folders.

Why?

• In both cases, it's unclear that they're actively used and useful
• In the case of fast_setup, it's unclear that it actually works, and has likely fallen out of sync with bin/setup
• In the case of fast_test, while this may have been helpful when accessibility tests were some of the only tests using ChromeDriver, there is now many more tests using the headless browser and therefore "fast_test" is not really very fast anyways.

📜 Testing Plan

Verify make fast_test and make fast_setup error due to removed task code.