bugfix: vote worker reached_end_of_slot set by is_complete

[apfitzge]

Problem

#7351 comment

Summary of Changes

• use is_complete() instead of !is_complete
• separate into function so we can test easily

Fixes #

[mergify]

Backports to the beta branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule. Exceptions include CI/metrics changes, CLI improvements and documentation updates on a case by case basis.

[apfitzge]

#7351 landed in 3.0, so adding backport label. Thanks to @diman-io for pointing this out.
Would have caused vote worker to do drain loops really inefficiently for packing votes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.2%. Comparing base (004ed58) to head (14e703f).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #8474     +/-   ##
=========================================
- Coverage    83.2%    83.2%   -0.1%     
=========================================
  Files         840      840             
  Lines      367918   367938     +20     
=========================================
- Hits       306145   306133     -12     
- Misses      61773    61805     +32     

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[tao-stones]

lgtm