Add stable throughput measurement test

It was observed that the HLT1 throughput varies significantly depending on which input sample is used, i.e. the events contained in upgrade_mc_minbias_scifi_v5_000 result in higher throughput(https://mattermost.web.cern.ch/lhcb/pl/hysg9hrej7b88bw1kn138qyuoc) than for example the ones in MiniBrunel_2018_MinBias_FTv4_DIGI_1k (https://mattermost.web.cern.ch/lhcb/pl/4jeap8j6n7ft8jp6pu3f6dn74a). The difference in throughput depends on the GPU card and can be as high as 20%. For the RTX2080Ti, no difference was observed.

This should be investigated further, and a throughput test running over both different sets of events (we only use the first 500 events at the moment) and with different input samples should be implemented.

For this, the option to start reading events with an offset has to be re-implemented.