Make use of RuleTest.expected

[forman]

Now making use of the expected property of RuleTest.

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

Attention: Patch coverage is 97.95918% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.59%. Comparing base (083b699) to head (1b29a69).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
xrlint/testing.py	97.95%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   97.52%   97.59%   +0.07%     
==========================================
  Files          62       62              
  Lines        2503     2535      +32     
  Branches      357      369      +12     
==========================================
+ Hits         2441     2474      +33     
+ Misses         31       30       -1     
  Partials       31       31              

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