[Autofill] Deflake AutofillInteractiveTestDynamicForm.FillCardOnReformattingForm

This CL deflakes an interactive UI test that waits for metrics that are
emitted asynchronously after voting has been finished.

To this end, this CL introduces new machinery to block a test until
voting is finished: we push a task on the task runner used for voting
and wait until it's finished using a RunLoop.

Bug: 1462103
Change-Id: Ie66a0d67ea7aae19b63c8ea8c9311670a1ebedc7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4700970
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Dominic Battre <battre@chromium.org>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1172998}