Fixed flaky SchedulerTest.Suppress.
Fixed flaky SchedulerTest.Suppress.

Root Cause: The reason is that the DECLINE call set filter as 1hr,
the Clock::advance set as 100m. A race condition is that both DECLINE
and SUPPRESS started up in different threads and the call Clock::advance
may be called before SUPPRESS finished. The clock advanced for 100m which
is greater than 1hr, this caused the allocator start to allocate resource
again and ASSERT_TRUE(event.isPending()) will be failed.

Solution: Make SUPPRESS call is finished before call Clock::advance

Review: https://reviews.apache.org/r/39548

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