Fix flakiness in some DumpAccessibilityTreeTests

The following CL disabled a few flaky dump tests :
https://chromium-review.googlesource.com/c/chromium/src/+/2645257

These appear to be flaking because accessibility events are not done
processing by the time the test exits, so the dump is captured before
the AXTree is stable for dumping.

Adding an event waiter for ax::mojom::Event::kEndOfTest and firing
SignalEndOfTest before the dump results are captured.

Bug: 1159660, 1169854, 1167331
Change-Id: I2723c1abcb248801e8b4873d3cd2da08d9c5b52c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2645043


Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Adam Ettenberger <Adam.Ettenberger@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#848821}