srp testing server: fix timeouts

fix, or find a workaround for the timeouts that happen when the twisted fake provider used in the srp tests is run by two different test modules. if run by separate, they run ok. when run together by the nose runner, the second instantiation times out.

(from redmine: created on 2013-04-02, closed on 2013-04-02, blocks #801 (closed))