Fix Ink2 toolbar tests not awaiting microtasks

There are some tests that need to wait for microtasks to complete before
proceeding, otherwise they will incorrectly succeed or fail in future
CLs. Add them.

To be specific, wait for microtasks to finish after drawing strokes and
dispatching events.

Change-Id: Ie856bde82a79083e90ddb33afd488383dbb69317
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5798096
Commit-Queue: Andy Phan <andyphan@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1345741}