auto_test_vs_ref crashes with warmup_rounds=0

auto_test_vs_ref does not handle the case when warmup_rounds=0.

Snippet of error:

            for evt in events:                                                                                                                                                                                     
                evt.wait()                                                                                                                                                                                         
            evt_start.wait()                                                                                                                                                                                       
            evt_end.wait()                                                                                                                                                                                        
                                                                                                                                                                                                                   
>           elapsed_event = (1e-9*events[-1].profile.END                                                                                                                                                           
                    - 1e-9*events[0].profile.START) \                                                                                                                                               
                    / timing_rounds                                                                                                                                                                                
E           IndexError: list index out of range                                                                                                                                                                    
                                                                                                                                                                                                                   
../../sw/loopy/loopy/auto_test.py:600: IndexError