Old kern writer: Convert groups back

[madig]

This fixes an oversight in https://github.com/googlefonts/ufo2ft/pull/841.

The kerning pair sides have to be converted back to fea AST classes. Since groups aren't filtered in getVariableKerningPairs, it should hopefully be okay to just grab them by their members 😬

[madig]

I don't understand the test failure... it werks on my local machine? Is there a accidental un-sorting problem hiding in there?

[anthrotype]

You could try re running the CI manually If it then passes we know there is some in-determinism hiding somewhere

[madig]

Ah yes, running it multiple times locally makes it crash occasionally. Let's see.

[anthrotype]

feel free to merge this when ready