Restore Config caching in tests.
Restore Config caching in tests.

It turns out that some tests still use it, subtly.
Things were working before because of the order in which
some tests happened to be run :(

TODO: Figure out that test interdependency issue.

Testing Done:
CI passed: https://travis-ci.org/pantsbuild/pants/builds/61229496.

Bugs closed: 1487

Reviewed at https://rbcommons.com/s/twitter/r/2160/

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