[tests] skip tests that need remote content if host is offline

[picnixz]

Don't forget people who work offline!

[jayaddison]

@picnixz test_run_epubcheck unexpectedly fails offline too, and could be worth adding to the list here.

The reason for that is quite obscure: it seems that we write a slightly different output .epub file containing 'remote references' if we're unable to download the content locally during the build. When epubcheck checks the (different) resulting output, it isn't happy about those remote references and complains.

I don't know of a way to run GitHub Actions workflows in a way that prevents them from making outbound network connectivity, but such a thing might exist.

[picnixz]

I actually don't run it because I don't have epubcheck.. so... I never saw the error.

[jayaddison]

Yep, neither had I previously - I'm trying to make the tests behave more similarly offline and online after seeing this change. In particular: replacing some of the remote image links with content served by our good friend localhost:7777.

When I did that, I tried to get as many skipped tests to run as possible (in case I'd missed any other cases), and found that test_run_epubcheck failed.. but only when offline! Weird.

[jayaddison]

Filed the details as #12094 - there are some easy-ish small improvements, and it may be low priority, but I'll try to suggest one or two adjustments soon.