Fix flaky ledger app tests that use commitRound directly

[algorandskiy]

Summary

The problem was in commitSyncer goroutine that triggered commitRound in additional to controlled commitRound calls.
Fixed by closing deferred commits channel and waiting commitSyncer to exit.

Test Plan

Checked with -count 1000

[codecov-commenter]

Codecov Report

Merging #2709 (69cb3f4) into master (39be0c7) will increase coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #2709      +/-   ##
==========================================
+ Coverage   47.07%   47.08%   +0.01%     
==========================================
  Files         349      349              
  Lines       55858    55858              
==========================================
+ Hits        26294    26301       +7     
+ Misses      26617    26611       -6     
+ Partials     2947     2946       -1     

Impacted Files	Coverage Δ
ledger/acctupdates.go	62.71% <50.00%> (+0.41%)	⬆️
ledger/blockqueue.go	83.90% <0.00%> (-1.15%)	⬇️
data/transactions/verify/txn.go	48.45% <0.00%> (ø)
network/requestTracker.go	71.12% <0.00%> (+0.86%)	⬆️
cmd/tealdbg/debugger.go	73.86% <0.00%> (+1.00%)	⬆️

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[tsachiherman]

Looks good to me