Testing: fix TestAttestorsChange

[id-ms]

Summary

TestAttestorsChange tries to compare account's balance with the state proof's total online stake.
The test sample the balance on a stateproof round (where the onlineStake field appears), However the balances should be sampled StateProofVotersLookback rounds before.

Test Plan

[codecov]

Codecov Report

Merging #4510 (8258d11) into master (eeeb7c3) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4510      +/-   ##
==========================================
- Coverage   55.30%   55.29%   -0.01%     
==========================================
  Files         398      398              
  Lines       50336    50336              
==========================================
- Hits        27840    27835       -5     
- Misses      20173    20174       +1     
- Partials     2323     2327       +4     

Impacted Files	Coverage Δ
agreement/cryptoVerifier.go	67.60% <0.00%> (-2.12%)	⬇️
agreement/proposalManager.go	96.07% <0.00%> (-1.97%)	⬇️
data/transactions/verify/txn.go	43.75% <0.00%> (-0.90%)	⬇️
ledger/acctupdates.go	69.59% <0.00%> (-0.60%)	⬇️
ledger/acctonline.go	78.12% <0.00%> (-0.53%)	⬇️
ledger/accountdb.go	72.87% <0.00%> (-0.16%)	⬇️
network/wsNetwork.go	65.10% <0.00%> (+0.28%)	⬆️
cmd/tealdbg/debugger.go	73.49% <0.00%> (+0.80%)	⬆️
ledger/tracker.go	77.87% <0.00%> (+2.97%)	⬆️

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