Skip test if it can not access /var/log/wtmp

utils/test/test-aa-notify.py:
Change `AANotifyTest.test_entries_since_login()` to be decorated by a
`skipUnless()` checking for existence of **/var/log/wtmp** (similar to
`AANotifyTest.test_entries_since_login_verbose()`).
The test otherwise fails trying to access /var/log/wtmp in environments
where the file is not available.

Fixes https://gitlab.com/apparmor/apparmor/-/issues/120
MR: https://gitlab.com/apparmor/apparmor/-/merge_requests/641
(cherry picked from commit e0200b1b16)
Signed-off-by: John Johansen <john.johansen@canonical.com>

utils/test/test-aa-notify.py

@@ -189,6 +189,7 @@ optional arguments:
         result = 'Got output "%s", expected "%s"\n' % (output, expected_output_has)
         self.assertIn(expected_output_has, output, result + output)
 
+    @unittest.skipUnless(os.path.isfile('/var/log/wtmp'), 'Requires wtmp on system')
     def test_entries_since_login(self):
         '''Test showing log entries since last login'''