Replace image diff tests with metrics tests

[forsyth2]

Request criteria

• I searched the zppy GitHub Discussions to find a similar question and didn't find it.
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Issue description

Replace image diff tests with metrics tests. See E3SM-Project/e3sm_diags#756 for more information.

[forsyth2]

@golaz pointed out that the image checker tests are quite useful for making sure no plots disappear (e.g., only showing 1 or 2 of the 3 plots on a comparison page in diags), as once happened. That said, it's possible just testing metrics could also accomplish this.