Make tests less sensitive to default kernel name

[blink1073]

The native kernel is faster, so we'd like to switch the default back in jupyter-server/pytest-jupyter#36. This change makes us less brittle.

[codecov]

Codecov Report

Base: 80.01% // Head: 80.04% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (a85405a) compared to base (118ac89).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1118      +/-   ##
==========================================
+ Coverage   80.01%   80.04%   +0.02%     
==========================================
  Files          68       68              
  Lines        8013     8013              
  Branches     1586     1586              
==========================================
+ Hits         6412     6414       +2     
+ Misses       1179     1178       -1     
+ Partials      422      421       -1     

Impacted Files	Coverage Δ
jupyter_server/serverapp.py	80.21% <0.00%> (+0.17%)	⬆️

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