Simplify test analytics

[martincostello]

• Only enable JUnit output in CI.
• Change the default JUnit file name so codecov/test-results-action finds the files automatically.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.23%. Comparing base (4dcfe05) to head (012ea23).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2596   +/-   ##
=======================================
  Coverage   96.23%   96.23%           
=======================================
  Files         311      311           
  Lines        7327     7327           
  Branches     1013     1013           
=======================================
  Hits         7051     7051           
  Misses        222      222           
  Partials       54       54           

Flag	Coverage Δ
linux	96.23% <ø> (ø)
macos	96.23% <ø> (ø)
windows	96.22% <ø> (ø)

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