Either fix internet_off or add a new decorator to disable internet

[drdavella]

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.

[drdavella]

It will be necessary to update the astropy testing documentation if/when this change is made.

[pllim]

This doesn't behave as expected in pytest 5.3 either. Works when I run it manually in interpreter but not when I invoke the test with pytest testfile. 🤷‍♀

with no_internet():
    something_that_downloads()