Fix flaky inactivity timer test

[mpass99]

Closes #669

There was a race condition between both channels in ChannelReceivesSomething having data.
The reason why this race condition appears only now is likely that the small delay of <-time.After(0) got reduced with the timer refactoring of Go 1.23.

[MrSerth]

Awesome finding, thanks! Happy to see that it only affects the tests, but not code we run in production. Thanks for taking care and linking it to Go 1.23

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.19%. Comparing base (e89ce3b) to head (3d79e0f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #670      +/-   ##
==========================================
- Coverage   76.28%   76.19%   -0.09%     
==========================================
  Files          43       43              
  Lines        3592     3592              
==========================================
- Hits         2740     2737       -3     
- Misses        623      625       +2     
- Partials      229      230       +1     

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