test: Fix goroutine leak in peer exchange exit

[kylecarbs]

Closes #361.

[codecov]

Codecov Report

Merging #370 (a29a032) into main (f630fc5) will increase coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #370      +/-   ##
==========================================
+ Coverage   67.46%   67.63%   +0.17%     
==========================================
  Files         148      148              
  Lines        8064     8064              
  Branches       72       72              
==========================================
+ Hits         5440     5454      +14     
+ Misses       2059     2048      -11     
+ Partials      565      562       -3     

Flag	Coverage Δ
unittest-go-macos-latest	66.24% <ø> (+0.14%)	⬆️
unittest-go-ubuntu-latest	67.32% <ø> (+0.29%)	⬆️
unittest-go-windows-2022	65.79% <ø> (+0.13%)	⬆️
unittest-js	66.10% <ø> (ø)

Impacted Files	Coverage Δ
provisionerd/provisionerd.go	69.92% <0.00%> (+0.58%)	⬆️
coderd/provisionerdaemons.go	60.80% <0.00%> (+0.59%)	⬆️
peerbroker/proxy.go	57.23% <0.00%> (+1.25%)	⬆️
peer/conn.go	77.49% <0.00%> (+1.53%)	⬆️

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[jawnsy]

seems reasonable 👍