Remove as many remote connections from tests as possible

[englehardt]

Nearly all of our tests require an external network connection to run. This is mostly a result of our choice to use localtest.me, which resolves to localhost.

I think we can get the same functionality by using the network.dns.localDomains pref in Firefox. It accepts a comma separate list of domains which, when set, will resolve to localhost. See this test as an example.

[vringar]

localtest.me doesn't resolve anymore for me, so the tests keep failing locally. It appears that they are partially working when run on GitHub Actions but I'm still going to prioritize replacing them.