Modernize the XRefMock unit-test helper

[Snuffleupagus]

No description provided.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.15%. Comparing base (f4a73b3) to head (5350280).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21782      +/-   ##
==========================================
- Coverage   90.16%   90.15%   -0.01%     
==========================================
  Files         264      264              
  Lines       67248    67285      +37     
==========================================
+ Hits        60635    60662      +27     
- Misses       6613     6623      +10     

Flag	Coverage Δ
unittest	58.27% <ø> (+<0.01%)	⬆️
unittestcli	56.81% <ø> (+<0.01%)	⬆️

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[timvandermeij]

r=me, with passing tests. Thank you!