Refactor test temp dir creation into its own class
Refactor test temp dir creation into its own class

Summary:
The logic for managing a test-local temporary directory is hidden inside
WatchmanTestCase's large interface. I want to use this logic in
test_nodejs.py's unittest.TestCase-based test, but the logic is not
directly reusable.

Move WatchmanTestCase's temp dir logic into its own mixin class. Have
WatchmanTestCase use the mixin to keep the interface the same as before
to avoid changing existing tests. (The mixin class will be used
elsewhere in a future commit.)

This commit should not change behaviour.

Reviewed By: wez

Differential Revision: D8431191

fbshipit-source-id: 2eea268c18ab28b6ce3c44bf4378b720ca595211

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