[Autofill] Fix TestAutofillManagerWaiter for pending irrelevant events

Suppose we have a TestAutofillManagerWaiter for OnFormsSeen().
Consider the following sequence of events:

1.  AM::OnFormsSeen()
1a. AM::Observer::OnBeforeFormsSeen()
1b. AM::Observer::OnAfterFormsSeen()
2.  AM::OnLanguageDetermined()
2a. AM::Observer::OnBeforeFormsSeen()
3.  TestAutofillManager::Wait(1)
4.  Foo(AM::form_structures())

The Wait(1) call is non-blocking because the relevant event,
OnFormsSeen(), has already happened. But OnLanguageDetermined()
temporarily borrows the AM::form_structures_ and returns them only
asynchronously. As a result, we unexpectedly call Foo({}).

This CL fixes that by letting Wait() block until all asynchronous
events, even irrelevant ones (i.e., non-OnFormsSeen() ones), have
finished (i.e., pending OnAfterFoo() have been called).

Bug: 357773190
Change-Id: If4ea3d2a04bbb7ad7a9276c353d822ec6dfbd53c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5805941
Commit-Queue: Christoph Schwering <schwering@google.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1345875}