test: Fix test flake panic in provisionerd

[kylecarbs]

Closes #382.

[codecov]

Codecov Report

Merging #383 (2758b1d) into main (8acd58a) will increase coverage by 0.53%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #383      +/-   ##
==========================================
+ Coverage   67.27%   67.81%   +0.53%     
==========================================
  Files         148      150       +2     
  Lines        8297     8475     +178     
  Branches       72       72              
==========================================
+ Hits         5582     5747     +165     
- Misses       2137     2149      +12     
- Partials      578      579       +1     

Flag	Coverage Δ
unittest-go-macos-latest	66.16% <ø> (?)
unittest-go-ubuntu-latest	67.39% <ø> (ø)
unittest-go-windows-2022	65.51% <ø> (?)
unittest-js	66.10% <ø> (ø)

Impacted Files	Coverage Δ
pty/pty_windows.go	60.31% <0.00%> (ø)
pty/start_windows.go	51.30% <0.00%> (ø)
peer/conn.go	77.74% <0.00%> (+0.76%)	⬆️
peerbroker/proxy.go	57.23% <0.00%> (+1.25%)	⬆️
provisionerd/provisionerd.go	68.56% <0.00%> (+1.28%)	⬆️
coderd/provisionerdaemons.go	57.82% <0.00%> (+1.60%)	⬆️
coderd/provisionerjobs.go	59.23% <0.00%> (+1.63%)	⬆️
coderd/workspacehistory.go	65.73% <0.00%> (+2.24%)	⬆️
provisioner/echo/serve.go	49.48% <0.00%> (+3.09%)	⬆️
coderd/workspaces.go	65.11% <0.00%> (+4.65%)	⬆️
... and 3 more

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[bryphe-coder]

Nice catch - great testing monday contribution!