bpo-32601: Let test_expanduser use the same user if no others found. (GH-5246)

This happens in the NixOS build sandbox, for example, where the only
other user is nobody with home directory /.
(cherry picked from commit 5c0d462689)

Co-authored-by: Anders Kaseorg <andersk@mit.edu>

Lib/test/test_pathlib.py

@@ -2142,6 +2142,9 @@ class PosixPathTest(_BasePathTest, unittest.TestCase):
             otherhome = pwdent.pw_dir.rstrip('/')
             if othername != username and otherhome:
                 break
+        else:
+            othername = username
+            otherhome = userhome
 
         p1 = P('~/Documents')
         p2 = P('~' + username + '/Documents')