Fixed a flaky test `MasterQuotaTest.RemoveSingleQuota`.
Fixed a flaky test `MasterQuotaTest.RemoveSingleQuota`.

The test is flaky due to a race between metrics update
and metrics query.

This patch adds clock settle to ensure quota update and
removal are fully processed (including metrics updates) before
continuing with the metrics query.

Review: https://reviews.apache.org/r/69981

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