test: make sure $NOTIFY_SOCKET is unset in test

When running the tests in Fedora's mock, the test would
fail because NOTIFY_SOCKET is set to /run/systemd/nspawn/notify, and
we get a permission error.

Author: keszybz

systemd/test/test_daemon.py

@@ -297,6 +297,8 @@ def test_listen_fds_with_names_multiple():
     assert listen_fds_with_names() == {}
 
 def test_notify_no_socket():
+    os.environ.pop('NOTIFY_SOCKET', None)
+
     assert notify('READY=1') is False
     with skip_enosys():
         assert notify('FDSTORE=1', fds=[]) is False