Tests: Fix t.Parallel() errors in shared package

[michaeldiamant]

Enables https://github.com/kunwardeep/paralleltest on shared by fixing linter warnings. Incrementally moves the ball towards greater unit test parallelization, which reduces local + CI test durations.

I vetted for flakiness by running:

go test -race -count 10 ./shared/...

[codecov]

Codecov Report

Merging #4989 (a2293ce) into master (eddf773) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4989      +/-   ##
==========================================
- Coverage   53.63%   53.63%   -0.01%     
==========================================
  Files         432      432              
  Lines       54057    54057              
==========================================
- Hits        28996    28995       -1     
- Misses      22812    22817       +5     
+ Partials     2249     2245       -4     

Impacted Files	Coverage Δ
ledger/blockqueue.go	85.63% <0.00%> (-2.88%)	⬇️
ledger/tracker.go	74.26% <0.00%> (-0.85%)	⬇️
ledger/acctupdates.go	69.24% <0.00%> (-0.25%)	⬇️
network/wsNetwork.go	64.74% <0.00%> (ø)
ledger/testing/randomAccounts.go	56.88% <0.00%> (+0.61%)	⬆️
catchup/service.go	70.53% <0.00%> (+0.72%)	⬆️
ledger/store/merkle_commiter.go	62.96% <0.00%> (+11.11%)	⬆️

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[algochoi]

There is only one test function that is being parallelized here, so the diff looks functionally equivalent to me. I'm okay with this change, but might be prudent for an extra approval.