[test][material-ui] Add tests for Pigment Grid and Stack

[DiegoAndai]

While reviewing #44042 I noticed we didn't have tests for these components. This PR adds them.

[mui-bot]

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[oliviertassinari]

I guess the next step (in the future) will be to move those to be demos on the docs. This way, we can see them in Netlify, Argos can take visual screenshots, etc.

[mnajdova]

I guess the next step (in the future) will be to move those to be demos on the docs. This way, we can see them in Netlify, Argos can take visual screenshots, etc.

@oliviertassinari we already have set up screenshot tests for Pigment CSS, it's using the fixtures directory from the vite app, check https://app.argos-ci.com/mui/material-ui/builds/32947/114088289

The main reason why I decided to use the vite app is because Pigment CSS is not used on the docs yet. We are anyway creating the whole docs page so all demos are duplicated & covered, but taken in one screenshot.