Add figure tests to CI

[samaloney]

Describe the feature

As we're now relying more on the figures we make it would be good to include figures tests. Should be able to essentially copy the configuration from sunpy and adapt as needed.

Proposed solution

No response

[hayesla]

@Amityush-lgtm, if you want to have a look at this?

[Amityush-lgtm]

since radiospectra generally follows SunPy's testing patterns, I assume using the same pytest-mpl workflow would be the preferred approach here but I wanted to check if that's the right direction before I start working on it.

[samaloney]

Yea the goal here is to detect unwanted changes the plots we generate should be able to use sunpy's figure test infrastructure as a guide.