add test for a_freq_j using cmh sato where tern bug occured

[iaugusty]

Pull Request

Fixes #nnn

Checks

• (Have you updated the NEWS.md ?)
• (Have you added proper tests for new functions/features ?)
• (Have you added new functions to the pkgdown.yml ?)
• (Have you run document() on new functions ?)

[wwojciech]

Thanks @iaugusty , good idea to have this test in place; but, no need to check s_proportion_diff() here, just check a_freq_j() only.

[wwojciech]

Looks, great, thanks @iaugusty for adding this!