forbid `only` in mocha tests on CI

[maribethb]

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Description

mocha has a flag --forbid-only we should use in CI so that tests fail if only is committed in a PR.

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[maribethb]

Unfortunately this is pretty complicated as we don't invoke mocha using the cli, we open an html page with webdriverio and run mocha in the browser, so it's not straightforward to pass in whether or not we're in CI environment.

You can't just always have the flag as true because it prevents any test from running and throws an error in the console, so it means you can't use only when you're debugging a test in development, which is extremely useful.

And even if you can pass down the ci env to set the flag appropriately, it breaks the error reporting and doesn't fail the test command correctly because it just throws an error in the browser console rather than updating the failureCount like it does for normal test failures. So that needs some work too.

It might be easier to do this with a lint-type check instead. Found this, will try it. https://github.com/lo1tuma/eslint-plugin-mocha/blob/main/docs/rules/no-exclusive-tests.md