#4938)

In #4919, we found that the `test_firefox` job was unacceptably flaky in
the `preload-cssom` tests. I believe this is because the GHA runners are
slightly more resource-constrained than the circleci ones, which broke
an assumption in `axe.testUtils.addStyleSheet` that adding a `<style>`
element would result in `document.styleSheets` being updated accordingly
within 100ms (in Chrome this happens instantly, but in Firefox there is
a delay). I updated the test util to poll until it actually *sees*
`document.styleSheets` update as expected, and to reject with a more
actionable error if a timeout occurs (rather than letting a test run in
an unexpected state). This also has a minor side-benefit of making the
tests marginally faster on Chrome, since they no longer wait
100ms-per-injected-sheet unnecessarily.

You can see the results of this in
#4936 - the preload-cssom
test_firefox case [only passed 6/10 runs without this
fix](https://github.com/dequelabs/axe-core/pull/4936/checks?sha=1516bacef0b7e039030adbd4d55c1a627f9899fb)
and [passed 10/10 runs with this
fix](https://github.com/dequelabs/axe-core/pull/4936/checks?sha=339c2580bb82f2f7833b8e2447bfeff92354350e).

The updates in `preload-cssom.js` turned out to be unnecessary to fix
the issue, but they make errors in the test more actionable and make the
test more consistent with others, so I left them in.

No QA required.