Add autotest shim from CPython 3.13.11

Lib/test/autotest.py

@@ -0,0 +1,5 @@
+# This should be equivalent to running regrtest.py from the cmdline.
+# It can be especially handy if you're in an interactive shell, e.g.,
+# from test import autotest.
+from test.libregrtest.main import main
+main()