[sidekiq] add basic Vernier spec

[solnic]

I started working on this in order to address some vernier/sidekiq issues, but these were addressed elsewhere eventually so we're left here with a spec that I wrote for vernier and sidekiq - I think it's beneficial to add it 🙂

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.05%. Comparing base (db975df) to head (5f338f1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2465      +/-   ##
==========================================
- Coverage   64.11%   64.05%   -0.07%     
==========================================
  Files         123      123              
  Lines        4724     4713      -11     
==========================================
- Hits         3029     3019      -10     
+ Misses       1695     1694       -1     

Components	Coverage Δ
sentry-ruby	59.84% <ø> (ø)
sentry-rails	59.32% <ø> (+0.10%)	⬆️
sentry-sidekiq	97.04% <ø> (-0.12%)	⬇️
sentry-resque	92.85% <ø> (ø)
sentry-delayed_job	95.65% <ø> (ø)
sentry-opentelemetry	99.31% <ø> (ø)

see 5 files with indirect coverage changes