Improve the performance of the obfs4 handshake test.

Exhaustively testing padding combinations is really slow, and was
causing timeouts during the Debian ARM package build process.  Attempt
to improve the situation by:

 * Reusing the client and server keypair for all of the tests, to cut
   runtime down by  ~50%.
 * Splitting the client side and server side tests up, as it appears
   the timeout is per-test case.

If this doesn't fix things, the next thing to try would be to reduce
the actual number of padding lengths tested, but that is a last resort
at the moment.