Tests: Remove unused newAcctUpdates parameter: dbPathPrefix

[michaeldiamant]

Summary

Makes these changes to newAcctUpdates:

• Remove unused parameter: dbPathPrefix. Since the method is test-only and all usages rely on ".", I'm guessing customization is not needed. If the preference is to use the parameter, let me know.
• Require no error during l.trackers.initialize. It seemed prudent to check the returned error.

I considered these changes while reviewing #4275. Consider the PR optional. If folks don't think it's worth pulling in, I'll happily close it.

Test Plan

gotestsum -- ./ledger

[codecov]

Codecov Report

Merging #4353 (6ee5542) into master (7a56ea7) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4353      +/-   ##
==========================================
+ Coverage   55.23%   55.26%   +0.02%     
==========================================
  Files         395      395              
  Lines       50352    50352              
==========================================
+ Hits        27811    27826      +15     
+ Misses      20149    20133      -16     
- Partials     2392     2393       +1     

Impacted Files	Coverage Δ
ledger/roundlru.go	90.56% <0.00%> (-5.67%)	⬇️
data/abi/abi_type.go	87.67% <0.00%> (-0.95%)	⬇️
data/transactions/verify/txn.go	44.00% <0.00%> (-0.89%)	⬇️
catchup/service.go	70.12% <0.00%> (ø)
network/wsNetwork.go	65.17% <0.00%> (+0.28%)	⬆️
ledger/acctupdates.go	70.52% <0.00%> (+0.60%)	⬆️
catchup/peerSelector.go	100.00% <0.00%> (+1.04%)	⬆️
util/db/dbutil.go	50.30% <0.00%> (+1.21%)	⬆️
ledger/tracker.go	78.63% <0.00%> (+1.28%)	⬆️
network/wsPeer.go	67.67% <0.00%> (+2.19%)	⬆️

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