Write test area file to tmp_path in rst list test

[djhoese]

test_area_def_rst_list previously wrote test_areas.yaml with a relative path, leaving a stray file in the current working directory.

Pure Claude after I noticed this test file showing up in my local directory.

• Closes #xxxx
• Tests added
• Tests passed
• Fully documented

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.90%. Comparing base (4a46786) to head (7c8ae3b).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #758   +/-   ##
=======================================
  Coverage   93.90%   93.90%           
=======================================
  Files          89       89           
  Lines       13924    13938   +14     
=======================================
+ Hits        13075    13089   +14     
  Misses        849      849           

Flag	Coverage Δ
unittests	93.90% <100.00%> (+<0.01%)	⬆️

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