Cache some external files downloaded during test runs

[claudep]

To avoid connectivity issues during running CI tests and save some bandwidth, it should be great if we could cache access to the SVG test suite and the Wikipedia flags. https://docs.github.com/en/actions/guides/caching-dependencies-to-speed-up-workflows has some tips for that.

@eumiro, do you think that would be feasible?

[deeplook]

Would make a lot of sense.

[eumiro]

Sounds like a good idea, I can have a look at that later. Did you try anything?

[claudep]

No, I trust experts :-)