Summary:
Unfortunately it looks like D6880256 was not sufficient to make
function_scheduler_test pass reliably on Travis CI builds.  This just disables
function_scheduler_test on open source CMake-based builds for now.

Ideally in the future we should try to convert most of the function scheduler
to use proper synchronization mechanisms (like folly::Baton) for synchronizing
between threads, rather than sleeping and expecting the other thread to have
completed its work before we wake up.

Reviewed By: yfeldblum

Differential Revision: D6891889

fbshipit-source-id: 9ee98f5807e1d90f083bd942ba98df6fad656ba6