This will attach the mock to window.LockScreen. But if you look at https://github.com/mozilla-b2g/gaia/blob/master/apps/system/js/nfc_manager.js#L58 and https://github.com/mozilla-b2g/gaia/blob/master/apps/system/js/nfc_manager.js#L68 you'll see that NfcManager is using window.lockScreen. This test fails with "ReferenceError: lockScreen is not defined". So for this test i would leave it as it is. 

It seems that window.lockScreen is obsolete and NfcManager should now use window.LockScreen instead. Let me know if you agree on that. If yes I'll file a separate bug for this, fix it and modify the test case.