gh-111495: add stub files for C API test modules (GH-111586)

This is to reduce merge conflicts (Modules/Setup.stdlib.in) for
subsequent pull requests for the issue.

Modules/_testcapi/bytes.c

@@ -0,0 +1,17 @@
+#include "parts.h"
+#include "util.h"
+
+
+static PyMethodDef test_methods[] = {
+    {NULL},
+};
+
+int
+_PyTestCapi_Init_Bytes(PyObject *m)
+{
+    if (PyModule_AddFunctions(m, test_methods) < 0) {
+        return -1;
+    }
+
+    return 0;
+}