Re-fixed many master allocator tests.
Re-fixed many master allocator tests.

When the slave has a very short lifetime, its scheduled registration
retry might occur when the test is tearing down. These unintuitively
motivated registrations in turn cause additional invocations of
`AddSlave` on the allocator.
Additionally, this also reverts the newly introduced Clock pauses as
they have shown to be problematic.

Review: https://reviews.apache.org/r/66165/

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