Remove duplicates of unifying test API.

[1uc]

There's three copies of unifying API for DataSet and Attribute. This removes two copies.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.28%. Comparing base (070badf) to head (67a571b).

:exclamation: Current head 67a571b differs from pull request most recent head 39dc73d. Consider uploading reports for the commit 39dc73d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #990      +/-   ##
==========================================
- Coverage   86.31%   86.28%   -0.03%     
==========================================
  Files          92       92              
  Lines        5919     5908      -11     
==========================================
- Hits         5109     5098      -11     
  Misses        810      810              

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