getdeps: export a property that contains the path to the output executable
getdeps: export a property that contains the path to the output executable

Summary:
Export a property that indicates the path to the test executable.  This is
useful for callers that want to install the binary or run it from other CMake
rules.

Reviewed By: simpkins

Differential Revision: D17647146

fbshipit-source-id: b32e2694e44a07d7c234e53a7a5c8443cb144487

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift