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!
Update the comment to reflect that this specifically doesn't cover www.example.*