Implement TestInterceptorNackReply

[jech]

Another end-to-end test, this time for the NACK responder. It verifies that we actually receive a reply to a NACK, both when we negotiate an RTX track and when we don't.

This currently fails, for unknown reasons.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.50%. Comparing base (1ee0299) to head (fb1c61f).

:exclamation: There is a different number of reports uploaded between BASE (1ee0299) and HEAD (fb1c61f). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (1ee0299)	HEAD (fb1c61f)
go	2	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2986       +/-   ##
===========================================
- Coverage   77.93%   63.50%   -14.43%     
===========================================
  Files          89       67       -22     
  Lines       10546     3957     -6589     
===========================================
- Hits         8219     2513     -5706     
+ Misses       1838     1316      -522     
+ Partials      489      128      -361     

Flag	Coverage Δ
go	?
wasm	63.50% <ø> (ø)

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