#461 implement json compare in tests

[John-Sharples]

Implements one example of comparing the json dump from a plotly.figure against expected output. A similar approach can be applied to other plots in future.

In this PR:

• move scatter plot data to test dir.
• add test for scatter plot
• add fixtures to sort and compare json data. This allows plotly images to be converted to json and tested against known output.

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[georgemccabe]

I'm not familiar enough with this logic to confidently review this PR, so I'll let @bikegeek review it.

[bikegeek]

The current scatter plot will be replaced by the refactored version in feature_23_scatter, but these changes can remain in place and replicated when feature_23_scatter is ready.

[John-Sharples]

closing this as changes already included in #466