Check if domain is proxied

[benbalter]

This adds a proxied? method to check if a domain is served by pages, but is proxied by a third-party service. If so, the Health Check shouldn't raise CNAME/A record errors.

The logic is as follows. A domain is proxied if:

1. It's served by a Cloudflare IP (existing logic)
2. The site returns GitHub server headers and it isn't:
   1. Pointed at a GitHub Pages IP, or
   2. Pointed at a GitHub Pages domain

(I also cleaned up some new test-related warnings that recently appeared by explicitly defining an expected error and explicitly vendoring test dependencies)

/cc @github/pages, @konklone

[benbalter]

/cc @jessephus again as an FYI for your /pages check usage. Once merged, in the event that a domain is served by Pages but proxied by a third party, the check will now indicate that.

[konklone]

👍 A good move to broaden the proxy check to be vendor-agnostic, and should address a few specific proxies I'm aware of.

[PaulSD]

👍

[benbalter]

This is now in production for GitHub Pages builds. Please holler if things are behaving as expected.