unit tests of callback_timer_locked - inconsistence with timer design

[KammutierSpule]

test_callback_timer_locked.cpp

It tests timer in a way that is not possible to be used.

    void callback2()
    {
      tick_list.push_back(ticks);

      p_controller->start(2);
      p_controller->start(1);
    }

callback2 calls start() (which has lock/unlock), while callback2 itself is called on a tick() (which is lock too). This would call a deadlock.

Relates to https://github.com/ETLCPP/etl/issues/952