gh-93649: Move _testcapi tests to specific files (#129544)

Move many functions from _testcapimodule.c into more specific files
in Modules/_testcapi/.

In moved code:

* Replace get_testerror() with PyExc_AssertionError.
* Replace raiseTestError() with
  PyErr_Format(PyExc_AssertionError, ...).

Modules/_testcapi/set.c

@@ -8,18 +8,37 @@ set_get_size(PyObject *self, PyObject *obj)
     RETURN_SIZE(PySet_GET_SIZE(obj));
 }
 
+
+static PyObject*
+test_set_type_size(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    PyObject *obj = PyList_New(0);
+    if (obj == NULL) {
+        return NULL;
+    }
+
+    // Ensure that following tests don't modify the object,
+    // to ensure that Py_DECREF() will not crash.
+    assert(Py_TYPE(obj) == &PyList_Type);
+    assert(Py_SIZE(obj) == 0);
+
+    // bpo-39573: Test Py_SET_TYPE() and Py_SET_SIZE() functions.
+    Py_SET_TYPE(obj, &PyList_Type);
+    Py_SET_SIZE(obj, 0);
+
+    Py_DECREF(obj);
+    Py_RETURN_NONE;
+}
+
+
 static PyMethodDef test_methods[] = {
     {"set_get_size", set_get_size, METH_O},
-
+    {"test_set_type_size", test_set_type_size, METH_NOARGS},
     {NULL},
 };
 
 int
 _PyTestCapi_Init_Set(PyObject *m)
 {
-    if (PyModule_AddFunctions(m, test_methods) < 0) {
-        return -1;
-    }
-
-    return 0;
+    return PyModule_AddFunctions(m, test_methods);
 }