Reachability tests for new obfs4 bridges

As a follow up to legacy/trac#29279 (moved), we can now set up some new reachability tests on a subset of the bridges we've gotten through our bridge campaign \o/

We probably don't want to test all of the new bridges in case these tests cause a bunch of bridges to get blocked when they otherwise wouldn't.

As mentioned in #29279:comment:9, we should sample bridges from our various distribution mechanisms (email, private, and HTTPS), and also from any finer grained partitions we have (email provider, subnet, etc.).