bpo-44860: Update test_sysconfig for posix_user platlib (GH-28235) (GH-28251)

Update test_sysconfig.test_user_similar() for the posix_user scheme:
"platlib" doesn't use sys.platlibdir.
(cherry picked from commit 49acac00c08838d8080ce00d02c05284b94f8fb2)

Co-authored-by: Victor Stinner <vstinner@python.org>
diff --git a/Lib/test/pythoninfo.py b/Lib/test/pythoninfo.py
index 278dfe7..39ee9e1 100644
--- a/Lib/test/pythoninfo.py
+++ b/Lib/test/pythoninfo.py
@@ -96,6 +96,7 @@ def collect_sys(info_add):
         'maxunicode',
         'path',
         'platform',
+        'platlibdir',
         'prefix',
         'thread_info',
         'version',