chore: Improve device test output

[jamescrosswell]

In the case of a failed test, the script now outputs any lines from test_output/TestResults.xml contining the text result="Fail", as a quick summary of any test failures.

Also, on CI, if there is no match for (Get-Item ./test_output/*.xml).FullName this resulted in an unrelated error (no such property FullName), which made it hard to identify the root cause. That code now only executes if there are some matching the pattern for the xml files.

#skip-changelog

[github-actions]

Semver Impact of This PR

⚪ None (no version bump detected)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.88%. Comparing base (7feb9a6) to head (61f066e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4997      +/-   ##
==========================================
- Coverage   73.89%   73.88%   -0.02%     
==========================================
  Files         496      496              
  Lines       17951    17951              
  Branches     3516     3516              
==========================================
- Hits        13265    13263       -2     
- Misses       3825     3826       +1     
- Partials      861      862       +1     

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