add tests for automatic naming of dimensions

[OriolAbril]

Add tests to guarantee we don't inadvertently change the naming scheme.

Note the naming scheme and autonaming is different from current ArviZ
as the assumptions about sample dimensions have changed a bit
(hopefully to make it easier to use with data that isn't chain, draw, *shape).

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📚 Documentation preview 📚: https://arviz-base--29.org.readthedocs.build/en/29/

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.57%. Comparing base (723044c) to head (0793ebe).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   59.40%   66.57%   +7.16%     
==========================================
  Files          14       14              
  Lines        1116     1116              
==========================================
+ Hits          663      743      +80     
+ Misses        453      373      -80     

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[amaloney]

lgtm