Develop unit tests

[alice-hannah]

Not particularly straight forward to test ggplot2 functions, but thinking we might be able to use ggplot2::build_plot() to check plot attributes

[alice-hannah]

https://vdiffr.r-lib.org/

[alice-hannah]

See afcharts PR - best-practice-and-impact/afcharts#37

[alice-hannah]

Testing use_sgplot will be easier when #20 closed