Refactored TemporaryDirectoryTest to be a mixin.
Refactored TemporaryDirectoryTest to be a mixin.

Currently you can't easily "mixin" the functionality of the
TemporaryDirectoryTest which makes for tests that might have a weird
inheritance structure. This refactor makes it easy for you to get the
functionality of the TemporaryDirectoryTest while easily composing it
with other test classes.

Review: https://reviews.apache.org/r/67957

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