Migrate from @open-wc/testing to e.g. @testing-library

[daneah]

The problem

Both @open-wc/testing and @open-wc/testing-helpers have not been published with regularity in the last couple of years. Recently, they have started degrading with the rest of the ecosystem, leading to the need to manage patches for them.

The solution

Move to a more widely-supported and current testing framework.

Alternatives considered

Worth looking into other available options, but Testing Library is a well-regarded favorite.

Additional information

Not to be confused with the parts we're using @web/test-runner for, we're just using @open-wc/testing for a few things specifically:

• fixture
• expect
• aTimeout
• nextFrame
• elementUpdated
• (probably accidentally since we usually import from 'lit/static-html.js', just a few instances of) html