add unit test for multiple mounts fsdetect
add unit test for multiple mounts fsdetect

Summary:
We intentionally choose to use the last matching entry in the
mount point to detect the type for repo watchman is trying to
watch. This is because this will be the type of mount that
shows up in the filesystem.

Let's add a test for this.

Reviewed By: chadaustin

Differential Revision: D37052907

fbshipit-source-id: 9ad015881221e9e7fee302cacbae929cc923e6a8

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