Implement a network-layer test harness for HTTPS Everywhere corectness

Since various changes to Mozilla internals break HTTPS Everywhere on a semi-regular basis, we should build a simple test harness that can watch the network for HTTP requests and automatically figure out whether any of them should have been rewritten by the ruleset library.

Known corner cases for this include URLs that redirect back to HTTP (these can be found out by watching the console output from Firefox with HTTPS Everywhere) and disabled rulesets.

But overall, this would be a very simple way to increase our confidence in HTTPS Everywhere's corectness as Mozilla's code changes.