Drop update desktop_notifications tests for now
    
    These tests don't work right at all at present: they don't test
    the update at all, they just boot the base image and run the
    test, which is stupid.
    
    I looked into various ways of fixing this but it's messy and I
    don't think it can work properly without a lot of hacking. Even
    if we get the test to do 'the right thing' - boot, set up the
    update repo, update, reboot, do the test prep, reboot again, do
    the actual test - I don't think it'll be quite a valid test,
    because I think any AVCs or crashes that happen *before* the
    update is installed will still appear as notifications when the
    test finally does log into the desktop. So the test can fail
    even if there are no post-update crashes or AVCs, I think.
    
    I decided to give up on trying to make this test work properly
    for now and just disable it. We can come back to it later if we
    have great ideas and/or lots of time...