Issue 918724  introduced a default timeout on RunLoop::Run() calls made on the main thread of any test that uses ScopedTaskEnvironment.  This could be improved by:

1. Reducing the default Run() timeout from action_max_timeout() to e.g. action_timeout().

2. Enabling Run() timeouts in base::TestSuite, so that tests that don't use ScopedTaskEnvironment still get them.

3. Enabling some form of timeout on Run[UntilIdle]() even when running under MOCK_TIME, to trap tests which run with mocked time that hang busy-looping over tasks.