[Material You] Update Button test & add active class name

[mnajdova]

I started the PR with the need to fix the active state for the Button, and then realized that in the test we were testing the @mui/material/Button, instead of @mui/material-next/Button, so I re-write the test and added some necessary features to the describeConformance test suit, mainly around supporting different createTheme feature.

[mui-bot]

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