chore: testing improvements

[EvanHahn]

This change makes several test-only improvements:

• prettier --check should be run. (I had to reconfigure Prettier a bit, and format CHANGELOG.md.)

• ESLint should be run with the --cache option.

• Test code should be run in parallel, for performance. ⚠️ This is controversial!

• lint-staged should run Prettier on all files, not just some extensions.

• lint-staged should only check files, not fix them. This addresses a race condition (search lint-staged's docs for "race condition").

[EvanHahn]

Made a mistake here. I also think it's better to split this into smaller PRs, which I'll do now.