Run unit tests for pluggable transports when building Tor Browser Bundle

dcf and I were recently discussing the dependencies between pluggable transports in the Tor Browser Bundle. As an example obfsproxy, flashproxy, and fteproxy all rely on pyptlib.

In these cases it is important that we have sanity checks to ensure that when one of these components is upgraded, it doesn't break any of the pluggable transports. It's tedious to manually verify that each pluggable transport works.

It appears that nearly all of the pluggable transports have unit tests. A simple sanity check, running all unit tests for all PTs during the build, is probably a good idea.