test_thread_state() doesn't check that the argument is a function and 
doesn't check function call result and so the exception is catched 
later:

Non callable argument:
   import _testcapi
   _testcapi._test_thread_state(10)
   # no exception raise here
   import gc
   # exception raised too late!
=> TypeError: 'int' object is not callable

Callback error:
   import _testcapi
   def err():
      raise ValueError("xxx")
   _testcapi._test_thread_state(err)
   # no exception raise here
   import gc
   # exception raised too late!
=> ValueError: xxx

So I wrote a patch which fixes that but also raise_exception() which 
have to check that the argument is an exception class.