Remove a negative test fails without tty

tests/test_cdiff.py

         os.chdir(self._cwd)
         self.assertEqual(ret, 0)
 
-    def test_read_diff_neg(self):
-        sys.argv = sys.argv[:1]
-        os.chdir(self._non_ws)
-        ret = cdiff.main()
-        os.chdir(self._cwd)
-        # Travis now runs tests with stdin closed
-        if sys.stdin.isatty():
-            self.assertNotEqual(ret, 0)
-
     def test_read_log(self):
         sys.argv = [sys.argv[0], '--log']
         self._change_file('read_log')