tests: make ledger tests ~20 sec faster

[algorandskiy]

Summary

1. Make TestCatchpointFileBalancesChunkEncoding smaller. It does not check limits now but only encode-decode trip.
2. Make TestLedgerMigrateV6ShrinkDeltas smaller.

Test Plan

These are tests

[codecov]

Codecov Report

Merging #4739 (241aa21) into master (fa77937) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4739      +/-   ##
==========================================
- Coverage   54.62%   54.57%   -0.05%     
==========================================
  Files         414      414              
  Lines       53514    53514              
==========================================
- Hits        29231    29207      -24     
- Misses      21860    21881      +21     
- Partials     2423     2426       +3     

Impacted Files	Coverage Δ
ledger/catchpointwriter.go	59.67% <ø> (ø)
ledger/blockqueue.go	85.63% <0.00%> (-2.88%)	⬇️
agreement/cryptoVerifier.go	67.60% <0.00%> (-2.12%)	⬇️
agreement/proposalManager.go	96.07% <0.00%> (-1.97%)	⬇️
network/wsPeer.go	66.50% <0.00%> (-1.95%)	⬇️
catchup/peerSelector.go	98.95% <0.00%> (-1.05%)	⬇️
cmd/tealdbg/debugger.go	72.69% <0.00%> (-0.81%)	⬇️
network/wsNetwork.go	65.34% <0.00%> (-0.37%)	⬇️
catchup/service.go	68.88% <0.00%> (ø)
ledger/testing/randomAccounts.go	56.61% <0.00%> (ø)
... and 1 more

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[cce]

LGTM, added a comment to help understand better what are the types inside the catchpoint chunks