Removing specs from finagle/finagle-native test.

Going along with the corresponding issue https://github.com/twitter/finagle/issues/290 and my other PR on the subject https://github.com/twitter/finagle/pull/292


There's no need for a Context class, you should just run the code inside `Context` before executing the test.
(I think this is the cause of the test failure on TravisCI)