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