Use custom sensitivity in tests for unstable counters

The following discussion from !349 (merged) should be addressed:

• @lhcbsoft started a discussion: (+3 comments)

  • [2020-01-18 00:09] Validation started with lhcb-gaudi-head#2506
  • [2020-01-19 00:14] Validation started with lhcb-gaudi-head#2507
  • [2020-01-20 00:09] Validation started with lhcb-gaudi-head#2508