watchman: resolve posix utilities from path in tests
watchman: resolve posix utilities from path in tests

Summary:
https://github.com/facebook/watchman/issues/811 raises an issue
on an unusual linux distro that doesn't put basic utilities in
`/bin`.  We should be fine to resolve these from the path so
adjust the test to do that.

Reviewed By: genevievehelsel

Differential Revision: D21478248

fbshipit-source-id: 508b846dfa0c05476d8962559b7a487d8bc1934d

nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman