race and linter fixes

[nonrational]

• error strings should not be capitalized or end with punctuation or a newline
• sort.Sort is in-place, so we shouldn't do it inside a request with a shared copy of http. instead, sort on main init, since the list of domains won't change without a restart.
• macOS' firewall is finicky about letting go bind and serve DNS inside tests, so leaning on GitHub CI to run on every push

note: I'm seeing this really weird behavior locally where make test backgrounds itself, so still trying to figure out if I somehow caused that with these changes or if it's an environmental thing. it's an environmental thing related to not have puma installed for the system ruby.