Either fix internet_off or add a new decorator to disable internet

Currently the `internet_off` decorator indicates that a test will only run if `--remote-data` is not provided or `--remote-data=none`. It would be more useful to have a decorator that actually disables network access for the scope of that test.

To accomplish this we could either update `internet_off`, or we could add a new decorator (maybe `disable_internet`). As far as I know, no one is using `internet_off` yet, so we probably don't have to worry about API breakage.