op-batcher: Fix test flake due to log

[mslipper]

There was one other spot where the batcher would log after tests exit, causing a panic.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 42.78%. Comparing base (9f84a4e) to head (1bdae2c).
Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
op-batcher/batcher/driver.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #13145      +/-   ##
===========================================
- Coverage    44.46%   42.78%   -1.68%     
===========================================
  Files          801      745      -56     
  Lines        72017    67356    -4661     
===========================================
- Hits         32021    28818    -3203     
+ Misses       37379    36110    -1269     
+ Partials      2617     2428     -189     

Flag	Coverage Δ
cannon-go-tests-32	?
cannon-go-tests-64	?

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Files with missing lines	Coverage Δ
op-batcher/batcher/driver.go	3.81% <0.00%> (-0.03%)	⬇️

... and 65 files with indirect coverage changes