Move test code out of xxmodule and into _testcapimodule.c where it belongs.

Will backport.
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 913c49a..6d8ea3c 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -522,6 +522,18 @@
 	return Py_None;
 }
 
+/* Example passing NULLs to PyObject_Str(NULL) and PyObject_Unicode(NULL). */
+
+static PyObject *
+test_null_strings(PyObject *self)
+{
+	PyObject *o1 = PyObject_Str(NULL), *o2 = PyObject_Unicode(NULL);
+	PyObject *tuple = PyTuple_Pack(2, o1, o2);
+	Py_XDECREF(o1);
+	Py_XDECREF(o2);
+	return tuple;
+}
+
 static PyObject *
 raise_exception(PyObject *self, PyObject *args)
 {
@@ -597,6 +609,7 @@
 	{"test_long_api",	(PyCFunction)test_long_api,	 METH_NOARGS},
 	{"test_long_numbits",	(PyCFunction)test_long_numbits,	 METH_NOARGS},
 	{"test_k_code",		(PyCFunction)test_k_code,	 METH_NOARGS},
+	{"test_null_strings",	(PyCFunction)test_null_strings,	 METH_NOARGS},
 
 	{"getargs_b",		(PyCFunction)getargs_b,		 METH_VARARGS},
 	{"getargs_B",		(PyCFunction)getargs_B,		 METH_VARARGS},