Add common ThreadingHelper module for specs

[mjankowski]

We have a bunch of specs with this same pattern of building an n-sized array of threads, waiting/joining them, running same code inside, asserting changes after.

This pulls out the thread setup/running portion of that pattern into reusable helper, and updates the relevant specs.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (86500e3) 85.01% compared to head (00ad2bf) 85.02%.
Report is 29 commits behind head on main.

Files	Patch %	Lines
app/controllers/api/v1/statuses_controller.rb	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #29116   +/-   ##
=======================================
  Coverage   85.01%   85.02%           
=======================================
  Files        1059     1059           
  Lines       28277    28292   +15     
  Branches     4538     4535    -3     
=======================================
+ Hits        24040    24054   +14     
- Misses       3074     3075    +1     
  Partials     1163     1163           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.