gh-105922: Add PyImport_AddModuleRef() function (#105923)

* Add tests on PyImport_AddModuleRef(), PyImport_AddModule() and
  PyImport_AddModuleObject().
* pythonrun.c: Replace Py_XNewRef(PyImport_AddModule(name)) with
  PyImport_AddModuleRef(name).

Modules/_testcapimodule.c

@@ -3325,6 +3325,53 @@ test_atexit(PyObject *self, PyObject *Py_UNUSED(args))
 }
 
 
+static PyObject *
+check_pyimport_addmodule(PyObject *self, PyObject *args)
+{
+    const char *name;
+    if (!PyArg_ParseTuple(args, "s", &name)) {
+        return NULL;
+    }
+
+    // test PyImport_AddModuleRef()
+    PyObject *module = PyImport_AddModuleRef(name);
+    if (module == NULL) {
+        return NULL;
+    }
+    assert(PyModule_Check(module));
+    // module is a strong reference
+
+    // test PyImport_AddModule()
+    PyObject *module2 = PyImport_AddModule(name);
+    if (module2 == NULL) {
+        goto error;
+    }
+    assert(PyModule_Check(module2));
+    assert(module2 == module);
+    // module2 is a borrowed ref
+
+    // test PyImport_AddModuleObject()
+    PyObject *name_obj = PyUnicode_FromString(name);
+    if (name_obj == NULL) {
+        goto error;
+    }
+    PyObject *module3 = PyImport_AddModuleObject(name_obj);
+    Py_DECREF(name_obj);
+    if (module3 == NULL) {
+        goto error;
+    }
+    assert(PyModule_Check(module3));
+    assert(module3 == module);
+    // module3 is a borrowed ref
+
+    return module;
+
+error:
+    Py_DECREF(module);
+    return NULL;
+}
+
+
 static PyMethodDef TestMethods[] = {
     {"set_errno",               set_errno,                       METH_VARARGS},
     {"test_config",             test_config,                     METH_NOARGS},
@@ -3468,6 +3515,7 @@ static PyMethodDef TestMethods[] = {
     {"function_get_kw_defaults", function_get_kw_defaults, METH_O, NULL},
     {"function_set_kw_defaults", function_set_kw_defaults, METH_VARARGS, NULL},
     {"test_atexit", test_atexit, METH_NOARGS},
+    {"check_pyimport_addmodule", check_pyimport_addmodule, METH_VARARGS},
     {NULL, NULL} /* sentinel */
 };