ci(pubsub): re-enable exactly-once test

[dbolduc]

Related to #5063

It looks like #3975 was the root problem for these acks failing. That is fixed, so re-enable the test, and see if we catch any other sort of error.

From longrunning benchmarks, I have only seen transient errors, which should be fixed by implementing the exactly once retry loop (#4804).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.98%. Comparing base (e92b011) to head (5bc82ac).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5262   +/-   ##
=======================================
  Coverage   97.98%   97.98%           
=======================================
  Files         215      215           
  Lines       44904    44904           
=======================================
+ Hits        43998    44000    +2     
+ Misses        906      904    -2     

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