[Autofill] Fix asynchronous parsing in AutofillManagerTest.

This CL is a speculative fix of flaky unit AutofillManagerTests when
asynchronous form parsing is enabled.

These tests parse a large number of forms. The parsing thread runs at
low priority. This CL simply increases the timeout of the run loop that
wait for the parsing.

Bug: 1309848, 1345089, 1360013
Change-Id: I0301b66970d56c58ee026198193725b532d9a2ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3888166


Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1045874}