Gossip: fix time-based tiebreak in test_refresh_vote_eviction

[alexpyattaev]

Problem

• Gossip can hold up to 12 votes, so when we push new votes we may need to evict old ones from CRDS.
• This eviction is timestamp-based to facilitate refresh votes (which may be cast for the same slot).
• test_refresh_vote_eviction ended up flaking due to identical timestamps, that resulted in the wrong items getting evicted. Failure was intermittent due to pubkey being part of the eviction tiebreak (which allowed it to pass sometimes).

Summary of Changes

• Add sleep between inserts to ensure deterministic operation
• This is not an issue in practice because we are not likely to switch votes within 1 ms in production environment

Context:

https://discord.com/channels/428295358100013066/560503042458517505/1483166748491518014

[alexpyattaev]

changed to lowest_vote_slot to match what the variable is actually set to a few lines above.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.2%. Comparing base (5b3a874) to head (47504f6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #11351   +/-   ##
=======================================
  Coverage    83.2%    83.2%           
=======================================
  Files         841      841           
  Lines      318444   318446    +2     
=======================================
+ Hits       265125   265138   +13     
+ Misses      53319    53308   -11     

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[gregcusack]

lgtm!