Run e2e tests across Chrome and Safari

For a test like https://github.com/mdn/fred/pull/1435 we ought to run cross-browser cross-OS, as text and scrollbars can and will render differently in those combinations.

We already run the npm simulation tests cross OS, but only in Firefox.

Our tests seem to fail locally for me in Chromium, due to some small differences in the playground copy test.

Our tests also will only run locally for me if I manually specify the path to my local Chromium binary. If I let wdio auto-download one, the tests seem to hang or never start.

Perhaps we don't need to run all tests cross-os cross-browser? Windows seems like a particularly slow path. But perhaps we go for coverage now, and optimize later.

We should also consider what versions to run against. Latest would be natural, but perhaps we should also run against the oldest versions in the Baseline browser set to ensure we don't break things there - we're least likely to test against those browsers after all.

Start by inspecting the existing npm simulation tests, the wdio setup, and the playground copy test that fails in Chromium. Run the current e2e tests in Chromium and review how Firefox is run across operating systems. Done means an agreed browser/OS/version matrix runs reliably, including Chrome and Safari, without the current startup or rendering failures.