`windows_host_engine_test` has gotten very flaky since thread merging

[matanlurey]

Commit: afdc861 (#163726).

See https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20Production%20Engine%20Drone:

Failures before (~0)

Failures after (~many)

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Example failures:

• https://cr-buildbucket.appspot.com/build/8717225845221013761
• https://cr-buildbucket.appspot.com/build/8717226941240514737
• https://cr-buildbucket.appspot.com/build/8717227996159337729
• https://cr-buildbucket.appspot.com/build/8717232472223529057
• https://cr-buildbucket.appspot.com/build/8717258058718895505
• https://cr-buildbucket.appspot.com/build/8717281729071274833
• https://cr-buildbucket.appspot.com/build/8717284529279395361
• https://cr-buildbucket.appspot.com/build/8717296510704283681

Some excerpts from above:

EmbedderTest.PlatformThreadIsolatesWithCustomPlatformTaskRunner
WindowsTest.LaunchMainHasNoOutput

I am marking this task as bringup: true and assigning to @knopp and @loic-sharma to investigate.