[RTADPA BW Tests] Prevent read_event_numbers wasting time counting events in files after reaching evtmax

read_event_numbers is used to evaluate the number of events present in the input files. i.e. the denominator for the rate (events processed) can be trivially min(events_present_in_input_files, events_requested)

However this is also a bit of a waste of time if you've ended up processing files with many more events than you requested. This trivially exists the cumulative count per file if it counts a value greater or equal to the evt_max.

Changed because I noticed in a log: Found 739396 events in input files (max of 300000), therefore n_evts in denominator of rates: 300000