tests: fix testCatchpointAfterStakeLookupTxns

[algorandskiy]

Summary

testCatchpointAfterStakeLookupTxns uses double ledger after adding lots of blocks, and then checks accounts are persisted on the same round. But sometimes it checks too early so validator has not persisted the latest block yet so flushing with testCatchpointFlushRound does flush but sometimes on a previous round.
Fixed by ensuring both ledgers have the latest block persisted.

Test Plan

This is a unit test fix

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.48%. Comparing base (6147ebf) to head (79b44a4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6363      +/-   ##
==========================================
- Coverage   50.66%   50.48%   -0.18%     
==========================================
  Files         653      653              
  Lines      110377   110377              
==========================================
- Hits        55918    55729     -189     
- Misses      51592    51788     +196     
+ Partials     2867     2860       -7     

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