build: upgrade @testing-library deps

[erikian]

Keeping up with RTL dependencies:

• @testing-library/react can't be upgraded because v12 is the last version that supports React 16 and 17, so we're stuck with that until we can upgrade React
• only a minor change was required for the @testing-library/jest-dom v5 -> v6 migration
• the latest major version of @testing-library/dom just works with the current setup
• the latest version of @testing-library/user-event is just a patch update for us

Ref: #1408

[coveralls]

coverage: 87.602%. remained the same
when pulling 5931ffb on build/update-rtl
into 3228c9c on main.

[dsanders11]

Thanks! Good to know about @testing-library/react being blocked by the React 18 upgrade.