Avoid testing duplication for packages zppy uses

[forsyth2]

Request criteria

• I searched the zppy GitHub Discussions to find a similar question and didn't find it.
• I searched the zppy documentation.
• This issue does not match the other templates (i.e., it is not a bug report, documentation request, feature request, or a question.)

Issue description

The zppy tests essentially test every package used by zppy in addition to zppy itself. What exactly should zppy testing focus on, as to not duplicate individual package tests?

Most notably, the image diff test is found in both E3SM Diags and zppy. Is it really necessary to have that test in both packages? Where does it really belong? (Note that there are plans to test metrics rather than image diffs: E3SM-Project/e3sm_diags#756, #536)