Test to make sure we isolate or disable all speculative connects

There are a variety of "resource hint" features in Tor Browser that we want to make sure are isolated by first-party or disabled. These include

  link rel=preconnect
  link rel=prefetch
  link rel=prerender

and possibly more. We should test this for the ESR45 and ESR52 versions of Tor Browser, because isolation will have different mechanisms.

See https://w3c.github.io/resource-hints/

We should also look into "SpeculativeConnect" code in Firefox to make sure there aren't any other cases of non-first-party isolated connections.