Bridge running flag is determined from bridgestrap tests, not bridge networks status

tpo/anti-censorship/team#112 (comment 2857497) claims we get the "running" flag from bridgestrap tests, which seems reasonable given the code linked to. However, the Onionoo documentation on the Metrics website says: "Boolean field saying whether this bridge was listed as running in the last bridge network status."

It seems we need to change the documentation here.