bpo-10496: posixpath.expanduser() catchs pwd.getpwuid() error (GH-10919)

* posixpath.expanduser() now returns the input path unchanged if
  the HOME environment variable is not set and pwd.getpwuid() raises
  KeyError (the current user identifier doesn't exist in the password
  database).
* Add test_no_home_directory() to test_site.