Teach Tor Browser How to [use Tor to] Test a Bridge

Roger said it this way:

basically what tor browser needs on the backend is a routine that tries a bridge+pt and decides whether it worked. and once we have that building block, many things get easier.

"given a PT and a bridge, try it and decide if bootstrapping worked"

This is important for creating a smart and usable Tor Browser. In fact, Orbot has had a similar feature (but not exactly the same) for two years.