Tests & CI

[ericphanson]

It seems this package doesn't have tests or CI. Could some be added? 😄

It is a dependency of Makie (cc @SimonDanisch) which has a lot of users so it would be good to have additional reassurance that it's dependencies all have tests & CI.

[kleinschmidt]

I think we could probably add some reference tests based on the front-end packages, to at least make sure that the basic behavior implemented here does not change (at least not without manually updating the reference values)

[pazner]

I don't currently have time to take this on, but happy to merge a PR if someone wants to propose something.