Replace ring_to_mat dictionary by example_rings vector

[fingolfin]

See #1902 -- I'd still like to get rid of the example_rings but at least the new code is closer to what I'd expect a successor in the conformance test suite to look like...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.17%. Comparing base (36951e9) to head (075afa5).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2076      +/-   ##
==========================================
- Coverage   88.18%   88.17%   -0.01%     
==========================================
  Files         100      100              
  Lines       36841    36840       -1     
==========================================
- Hits        32488    32485       -3     
- Misses       4353     4355       +2     

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