Investigate getProcessCpuTime usage in acceptance tests

[cka-y]

Food for thought: I wonder if we could use getProcessCpuTime to get amore accurate time estimate, since AFAIK it gives the actual time used by the process (ie without interference from other processes that are sharing the CPU). Just an idea, I never used this myself.

Originally posted by @jcpitre in https://github.com/MobilityData/gtfs-validator/pull/1771#discussion_r1616402082