functions.  I intend to replace their guts with calls to the new
_PyLong_{As,From}ByteArray() functions, but AFAICT there's no tests for
them at all now; I also suspect PyLong_AsLongLong() isn't catching all
overflow cases, but without a std test to demonstrate that why should you
believe me <wink>.

Also added a raiseTestError() utility function.