test: simplify TestWebsocketNetworkMessageOfInterest

[brianolson]

Summary

two calls to RegisterMessageInterest() was just a little too hard to synchronize and addedd a rare race bug to the test

Test Plan

Ran test several thousand times and old race condition did not recur (previously reproduced in ~400 runs)

[codecov]

Codecov Report

Merging #4341 (4fe8e74) into master (f15b3a4) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4341      +/-   ##
==========================================
+ Coverage   55.25%   55.27%   +0.01%     
==========================================
  Files         395      395              
  Lines       50341    50341              
==========================================
+ Hits        27814    27824      +10     
+ Misses      20137    20121      -16     
- Partials     2390     2396       +6     

Impacted Files	Coverage Δ
ledger/tracker.go	77.15% <0.00%> (-1.73%)	⬇️
catchup/service.go	68.88% <0.00%> (-1.24%)	⬇️
crypto/merkletrie/node.go	90.69% <0.00%> (-0.94%)	⬇️
network/wsNetwork.go	64.70% <0.00%> (ø)
ledger/acctupdates.go	70.52% <0.00%> (+0.60%)	⬆️
catchup/peerSelector.go	100.00% <0.00%> (+1.04%)	⬆️
node/node.go	25.39% <0.00%> (+2.38%)	⬆️

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