Be more efficient when testing bridges

There's room to make bridgestrap more efficient when it tests bridge lines. Roger helpfully suggested the following:

• Reuse Tor's data directory. (Bridgestrap currently deletes it after a test).
• Consider running a single Tor client and use SETCONF to test different bridges.
• If we can fetch the bridge's descriptor, we know that 1) we can establish a handshake and 2) the bridge knows its address. If we do a full bootstrap instead, we also learn if the bridge can properly fetch and serve directory information too.