llmainthreadtask_test builds in a Sync timeout to keep build-time tests from
hanging. That timeout was set to 2000ms, which seems as though it ought to be
plenty enough time for a process with only 2 threads to exchange data between
them. But on TeamCity EC2 Windows build hosts, sometimes we hit that timeout
and fail. Extend it to try to improve the robustness of builds, even though
the possibility of a production viewer blocking for that long for anything
seems worrisome. (Fortunately the production viewer does not use Sync.)

Now that we have the Sync class to help construct unit tests that move forward
in a deterministic stepwise order, we can build suitable unit tests for
LLMainThreadTask.