Test fix: flaky TestStreamVerifierCtxCancelPoolQueue

[algonautshant]

The test expects to have the stream verifier shut down after the EnqueueBacklog returns an error.
To induce an error from EnqueueBacklog, the tests cancels the task ctx while the task is in the exec pool queue.

However, the task ctx canceled is the same as the stream verifier ctx, and occasionally, the stream verifier shuts down because of the ctx before it gets the change to enqueue the task to EnqueueBacklog. When this happens, the tests fails, because the expected message does not get logged.

To fix this, never shut down the stream verifier ctx, instead, shut down the verification pool ctx to make sure the batch is rejected from the exec pool and not the batch ctx.

[codecov]

Codecov Report

Merging #4980 (b676ef4) into master (a35f4b8) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4980      +/-   ##
==========================================
+ Coverage   53.67%   53.69%   +0.02%     
==========================================
  Files         432      432              
  Lines       54077    54077              
==========================================
+ Hits        29025    29036      +11     
+ Misses      22806    22797       -9     
+ Partials     2246     2244       -2     

Impacted Files	Coverage Δ
ledger/roundlru.go	90.56% <0.00%> (-5.67%)	⬇️
network/wsNetwork.go	64.74% <0.00%> (ø)
catchup/peerSelector.go	100.00% <0.00%> (+1.04%)	⬆️
ledger/blockqueue.go	88.50% <0.00%> (+2.87%)	⬆️
ledger/tracker.go	78.05% <0.00%> (+2.95%)	⬆️

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