DM-45432: Add explicit tests for regions and timespans with query_datasets

[timj]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.47%. Comparing base (20b5ab1) to head (c255a5c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1049      +/-   ##
==========================================
+ Coverage   89.44%   89.47%   +0.03%     
==========================================
  Files         360      360              
  Lines       45842    45879      +37     
  Branches     9406     9410       +4     
==========================================
+ Hits        41004    41052      +48     
+ Misses       3506     3496      -10     
+ Partials     1332     1331       -1     

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