v3.0: bugfix: vote worker reached_end_of_slot set by is_complete (backport of #8474)

[mergify]

Problem

#7351 comment

Summary of Changes

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

Fixes #

---

This is an automatic backport of pull request #8474 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 904be9f has failed:

On branch mergify/bp/v3.0/pr-8474
Your branch is up to date with 'origin/v3.0'.

You are currently cherry-picking commit 904be9f96.
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Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   core/src/banking_stage/vote_worker.rs

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.5%. Comparing base (60b612f) to head (80517e7).
⚠️ Report is 36 commits behind head on v3.0.

Additional details and impacted files

@@           Coverage Diff           @@
##             v3.0    #8481   +/-   ##
=======================================
  Coverage    83.5%    83.5%           
=======================================
  Files         810      810           
  Lines      365988   366008   +20     
=======================================
+ Hits       305607   305626   +19     
- Misses      60381    60382    +1     

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[t-nelson]

can we get rid of all this fuckin' c in master?

https://github.com/anza-xyz/agave/pull/8481/files#diff-adaec8b3abe01d14df2eca34bcf3ef65f42862cf551f8250211cc9aca7cfb869R266-R276

sry gh too dumb to unfold the context. here

agave/core/src/banking_stage/vote_worker.rs

Lines 209 to 216 in 80517e7

	fn process_packets(
	&mut self,
	bank: &Bank,
	consumed_buffered_packets_count: &mut usize,
	rebuffered_packet_count: &mut usize,
	banking_stage_stats: &BankingStageStats,
	slot_metrics_tracker: &mut LeaderSlotMetricsTracker,
	) -> bool {