Add an end-to-end test for the NACK sender.

[jech]

End-to-end test for the NACK sending interceptor. It notably tests negotiation. See the function comment for details.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.87%. Comparing base (92d573c) to head (3b2ae1f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2984      +/-   ##
==========================================
- Coverage   77.88%   77.87%   -0.01%     
==========================================
  Files          89       89              
  Lines       10524    10524              
==========================================
- Hits         8197     8196       -1     
  Misses       1838     1838              
- Partials      489      490       +1     

Flag	Coverage Δ
go	79.45% <ø> (-0.02%)	⬇️
wasm	63.45% <ø> (ø)

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