Add support for variant

[nilsreichardt]

The testWidgets method allows specifying variant parameter. This is helpful when you have different design for different platforms.

[NickalasB]

HI @nilsreichardt. That doesn't seem like too crazy of a change. Would you be interested in opening up a PR to add the variant param to the testGoldens function?

[nilsreichardt]

There is already an open PR. But the author seems to be inactive. Should I create a new PR or write a comment under the PR?

[NickalasB]

Yeah I think making a new PR that is limited to just adding the variant param would be easiest honestly