Enhancements to MCB detection tests

@micahflee wrote these live-browser tests to find rulesets that trigger Firefox's Mixed Content Blocker (MCB).

This pull request improves on those tests in the following ways:

* don't test a ruleset if it is currently disabled on Firefox (reduces noise and speeds up the tests)
* begin each test at http://example.com/ rather than https://example.com/, since the ruleset might not rewrite the root path
* fix a bug where we would inadvertently try to test http://*.example.com when rulesets target it
* in those cases, test http://www.example.com if we weren't going to be doing that anyway

Targetting this branch at 4.0 since we should do maximum testing on stable rulesets first!
I think it's redundant to do the 't in targets_to_ids' check here, since addTestTargets should deal correctly with duplicates anyhow (since dupes across rulesets are allowed).