Replace PhantomJS for headless browser testing

[quincylvania]

PhantomJS was abandoned in 2018 and the mocha-phantomjs-core project used by iD for testing followed suit. We've been coasting until now, but mocha just deprecated some of the functions used by mocha-phantomjs-core (#7695). This means we can't keep up-to-date with mocha with our current setup, which is an unsustainable proposition. Testing is fundamental.

There's been talk in the past of switching away from PhantomJS, but now it's become a more immediate priority. I'm sure there are good, modern solutions out there, but we need to make sure we're still testing legacy browsers where possible. PhantomJS uses the same polyfills as Internet Explorer and as a result, tests for compatibility issues that might otherwise be difficult to find. See #6087 for related work.

[HolgerJeromin]

Can this issue be closed?