Fix base_services_start
    
    The old version wasn't working - it was passing even though two
    services fail to start in Workstation currently. I'm really not
    sure why the old approach wasn't working, but it wasn't, and I
    rather hate `script_output` anyway, so here's a different way
    of doing it which relies on `eval`ing `assert_script_output`
    instead. (I really should send a PR for a non-fatal version of
    assert_script_output...)