Restore mocha as a test runner option for easier dev debugging

[botandrose]

Switching our test suite from mocha-chrome to web-test-runner has been a big win for CI:

• We're generating code coverage metrics without ceremony
• We're testing in all major browsers
• We're no longer using unmaintained software!

However, when debugging locally, web-test-runner has some tedious downsides compared to mocha:

• We have to run npm run debug in order to load the test
• We have to annotate the test with it.only if we want to run a single test
• We have to close the browser and rerun npm run debug each time we make a change
• We have to remember to do a hard refresh each time or we'll get stale versions of the content

This PR restores mocha to the testing situation as an additional option, purely for use in local debugging, so we can just open test/index.html in a browser like before! Of course, there are some caveats:

• Perf tests are not available in mocha, due to their use of fetch, which is incompatible with the file:// protocol
• There is now some duplication between the html in test/index.html and web-test-runner.config.mjs
• Mocha is not run in CI, so if the config for it breaks, we won't know until we run it manually
• We have to ensure tests remain compatible with both frameworks

I can imagine ways to address most of these caveats, but they're out of scope for this PR.

I plan to exercise my commit bit and merge this myself if there aren't any objections, so this PR is mostly a FYI for @1cg and @MichaelWest22, but please do comment if you have any feedback!

[1cg]

awesome, really like that dev workflow, makes debugging a lot easier in my experience (btw, you can run a webserver from the command line pretty easily python3 -m http.server 8080)

i use WebStorm which has an integrated web server so I can right click on it and open it up in a web server really efficiently

[botandrose]

Awesome, I didn't know Python made it that easy! That's a super handy tool for the toolbelt.

Turns out Ruby can do it too: ruby -run -e httpd