De-flake AccessibilityEventsAriaComboBoxExpand test for IA2

This test has been flaky since (at least) last year: Sometimes two events
are fired on an object with ROLE_SYSTEM_CARET; sometimes they are not.
Because these two events are not critical to what we're attempting to
test, do not include them as expected events.

AX-Relnotes: n/a

Bug: 1321954
Change-Id: Ib9b033e0708be85b342df4b17d1276cf52b80b33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3629410


Auto-Submit: Joanmarie Diggs <jdiggs@igalia.com>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#999895}