Add a stable attribute in api_maturity.xml test and fix up tests

[andy31415]

gen-tests has hardcoded indices for attribute comparisons. Adding more attributes for testing
would make tests fail.

This is a stand-alone change that adds an attribute to the maturity test cluster and fixes up the test to not
rely on ordering (or count for that matter).

This is in preparation of having maturity flags on attributes, which would require me to add more attributes to the test XML.

[codecov-commenter]

Codecov Report

Merging #1168 (2945056) into master (318db74) will not change coverage.
The diff coverage is n/a.

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##           master    #1168   +/-   ##
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  Coverage   65.41%   65.41%           
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  Files         171      171           
  Lines       18384    18384           
  Branches     3971     3971           
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  Hits        12025    12025           
  Misses       6359     6359