How does the clock work in rostest

edit

I have built a very simple test, something like:

#include <ros/node_handle.h>
#include <gtest/gtest.h>

struct SimpleTicker {

    SimpleTicker(ros::NodeHandle &nh) : _nh(nh) {
       _stateTickTimer = _nh.createTimer(ros::Duration(0.25f),
      &SimpleTicker::incrementVal, this, false, true);
    }

    void incrementVal(const ros::TimerEvent&) {
        val++;
    }

    int val = 0;
    ros::Timer _stateTickTimer;
    ros::NodeHandle _nh;

};


TEST(SlamManagerTest, run_simple_tick) {
    ros::NodeHandle nh(~);
    SimpleTicker s(nh);
    ros::Duration{1}.sleep();
    ASSERT_GT(s.val, 0);
}


int main(int argc, char **argv) 
{
    testing::InitGoogleTest(&argc, argv);
    ros::init(argc, argv, "simple_timer_test");
    ros::NodeHandle nh;
    if (ros::console::set_logger_level(ROSCONSOLE_DEFAULT_NAME, 
                                       ros::console::levels::Debug))
    {
        ros::console::notifyLoggerLevelsChanged(); // show debug output in tests
    }
    return RUN_ALL_TESTS();
}

This extremely simple test, starts a class with a timer that should call back to increment a value every 0.25 seconds. I launch this with the test file:

<launch>
  <test test-name="simple_timer_test" pkg="manager" type="simple_timer_test">
  </test>
</launch>

But the test fails and the value is never incremented. I suspect something is wrong with how I setup rostest to use the clock. Any ideas about this?