testing: fix unit test data race

[tsachiherman]

Summary

The unit test TestLatestSigsFromThisNode was data racing. The fix is trivial.

Test Plan

This is a test.

[codecov-commenter]

Codecov Report

Merging #2926 (19ffe30) into master (dbab85f) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2926   +/-   ##
=======================================
  Coverage   47.29%   47.29%           
=======================================
  Files         356      356           
  Lines       57240    57240           
=======================================
+ Hits        27071    27072    +1     
  Misses      27103    27103           
+ Partials     3066     3065    -1     

Impacted Files	Coverage Δ
cmd/algoh/blockWatcher.go	77.77% <0.00%> (-3.18%)	⬇️
ledger/blockqueue.go	82.18% <0.00%> (-2.88%)	⬇️
catchup/service.go	68.57% <0.00%> (-1.56%)	⬇️
ledger/acctupdates.go	62.13% <0.00%> (-0.42%)	⬇️
cmd/tealdbg/debugger.go	73.86% <0.00%> (+1.00%)	⬆️
catchup/peerSelector.go	100.00% <0.00%> (+1.04%)	⬆️
crypto/merkletrie/node.go	93.48% <0.00%> (+1.86%)	⬆️
crypto/merkletrie/trie.go	68.61% <0.00%> (+2.18%)	⬆️
network/wsPeer.go	74.37% <0.00%> (+2.22%)	⬆️

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[algonautshant]

Looks good.