Tests: Fix flakey ECDSA testing

[jasonpaulos]

Summary

Fix flakey ECDSA testing, where a signature component had a leading zero byte, so big.Int.Bytes() produced a 31 byte slice.

The failing job: https://app.circleci.com/pipelines/github/algorand/go-algorand/18380/workflows/1d84e0b5-2ad9-4ac7-94cf-5c6315ac7138/jobs/272246

Test Plan

Test only fix

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.87%. Comparing base (2335b35) to head (59ad02b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6005      +/-   ##
==========================================
+ Coverage   55.86%   55.87%   +0.01%     
==========================================
  Files         481      481              
  Lines       68357    68357              
==========================================
+ Hits        38185    38193       +8     
+ Misses      27568    27563       -5     
+ Partials     2604     2601       -3     

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