add generic image test caching

[bjlittle]

🚀 Pull Request

Description

This pull-request unifies plotting image test caching.

The previous approach (removed in #822) was only appropriate for image testing of the geovista.examples and did not easily extent to the changes introduced by #819, which expanded image testing beyond geovista.examples.

Reference:

• https://github.com/bjlittle/geovista-data/pull/56

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[codecov[bot]]

Codecov Report

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

Project coverage is 91.18%. Comparing base (b52ec46) to head (ccf651c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #825 +/- ## ======================================= Coverage 91.18% 91.18% ======================================= Files 58 58 Lines 2939 2939 ======================================= Hits 2680 2680 Misses 259 259 ```

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