message: Add convenience function to the test actor clock
diff: @@ -49,6 +49,13 @@ public:
  /// @returns The number of triggered timeouts.
  size_t advance_time(duration_type x);
  + // -- properties -------------------------------------------------------------
  +
  + /// @pre has_pending_timeout()
  + time_point next_timeout() const {
  + return actions.begin()->first;
  + }
  +
  // -- member variables -------------------------------------------------------
  time_point current_time;
  