message: Set MM backend to testing in testing DSL
diff: @@ -580,7 +580,8 @@ struct test_coordinator_fixture {
  explicit test_coordinator_fixture(Ts&&... xs)
  : cfg(std::forward<Ts>(xs)...),
  sys(cfg.parse(caf::test::engine::argc(), caf::test::engine::argv())
  - .set("scheduler.policy", caf::atom("testing"))),
  + .set("scheduler.policy", caf::atom("testing"))
  + .set("middleman.network-backend", caf::atom("testing"))),
  self(sys, true),
  sched(dynamic_cast<scheduler_type&>(sys.scheduler())),
  credit_round_interval(cfg.streaming_credit_round_interval()),
  