Tests for plotting

[oameye]

As the recent update showed it could be handy to have some testing function for the plotting capabilities of HarmonicBalance. However, not sure how to test plotting other than testing if the plot funtion run.

[oameye]

So one can test the plot functionality by using VisualRegressionTests.jl of the Plots.jl ecosystem. If I understand correctly, one can use the package to test without adding it to the dependencies by adding it to extras in the project.toml file. @jkosata Can you confirm?

Sources:

• https://discourse.julialang.org/t/how-to-test-plot-recipes/2648/3

[oameye]

So I actually think this is maybe a bit of an overkill 😛. I think the basic running test that were implemented in #88 are enough for a package of our scale.