make Watchman integration tests twice as fast on my devserver by respecting TMPDIR
make Watchman integration tests twice as fast on my devserver by respecting TMPDIR

Summary:
TMP is nonstandard, so use the POSIX TMPDIR variable instead when
determining where to put repos for the Watchman integration tests.

Reviewed By: fanzeyi

Differential Revision: D34285595

fbshipit-source-id: 427d67bca747ad526b7a5d6b93220d1a63fe9300

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