This commit replaces `waitForFunction` calls that use
`document.activeElement` to wait for an element to get focus by simpler
`waitForSelector` expressions that use the `:focus` selector. Note that
we already use this in other tests, so this improves consistency too.
This commit replaces all `waitForTimeout` occurrences with the
appropriate `waitForFunction` calls. Note that in some places they were
already present, so in those cases we could simply remove the
`waitForTimeout` call altogether.
The `waitForTimeout` function should not be used anymore and only exists
for old usages that have to be rewritten, but there was nothing in place
to signal this. This commit therefore implements a linting rule, specific
to the integration tests, to make it clear that this function should no
longer be used. We exclude the old usages from it because we are already
tracking those in #17656 (so this patch is mostly to not make the scope
of that issue bigger).
