cmake: mark the async_test as BROKEN for now
cmake: mark the async_test as BROKEN for now

Summary:
Disable running the async_test in open source builds.  A few of the tests
functions in this binary are flaky.

Additionally, this test consistently hangs in Travis CI builds.  It hangs in
`AsyncSocketTest.ServerAcceptOptions`; I suspect it never sees the socket
accept event and therefore never breaks out of the loop.  I suspect that even
if we fixed this test other subsequent tests would also hang.  We should
ideally update all of the tests to always have their own internal event base
loop timeout rather than potentially looping forever, so it could at least run
other test functions after a hang like this.

Reviewed By: yfeldblum

Differential Revision: D6874011

fbshipit-source-id: 46c07a1cf3216eda1900e214447e6e49b6c8e489

facebook/folly,facebook/folly,facebook/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,rklabs/folly,rklabs/folly,rklabs/folly