ansible: fix test failure during process exit.

======================================================================
ERROR: tests.connection_test (unittest2.loader._FailedTest)
----------------------------------------------------------------------
Traceback (most recent call last):
ImportError: Failed to import test module: tests.connection_test
Traceback (most recent call last):
  File "/home/dmw/src/mitogen/.venv/local/lib/python2.7/site-packages/unittest2/loader.py", line 456, in _find_test_path
    module = self._get_module_from_name(name)
  File "/home/dmw/src/mitogen/.venv/local/lib/python2.7/site-packages/unittest2/loader.py", line 395, in _get_module_from_name
    __import__(name)
RuntimeError: not holding the import lock

ansible_mitogen/process.py

@@ -194,7 +194,10 @@ class MuxProcess(object):
             cls.worker_sock = None
             self = cls()
             self.worker_main()
-            sys.exit()
+            # Test frameworks living somewhere higher on the stack of the
+            # original parent process may try to catch sys.exit(), so do a C
+            # level exit instead.
+            os._exit(0)
 
     def worker_main(self):
         """