[3.9] bpo-43517 (followup): Install additional test directory (GH-24950) (GH-24951)

Should fix some CI failures on buildbots that test an installed version of Python.


(cherry picked from commit 66c8adf)

Co-authored-by: Antoine Pitrou <antoine@python.org>

Author: pitrou

Makefile.pre.in

@@ -1439,6 +1439,7 @@ LIBSUBDIRS=	tkinter tkinter/test tkinter/test/test_tkinter \
 		test/test_importlib/namespace_pkgs/project3 \
 		test/test_importlib/namespace_pkgs/project3/parent \
 		test/test_importlib/namespace_pkgs/project3/parent/child \
+		test/test_importlib/partial \
 		test/test_importlib/source \
 		test/test_importlib/zipdata01 \
 		test/test_importlib/zipdata02 \