Test: Add inheritance tests

[NickDJM]

Summary

There's a lot of prop inheritance going on in Grau and I think it would be pretty cool if we supported some testing to make sure things inherited properly.

We'd need to test changing root-level variables and component-level variables. Basically everything along the chain.

This would essentially be unit tests in a way.

Example:

Test buttons border radius.

1. If you change the --graupl-border-radius prop to 2rem, the buttons border radius should be 2rem.
2. If you change the --graupl-button-border-radius prop to 2rem, the buttons border radius should be 2rem.
3. If you change the --graupl-border-radius prop to 2rem and the --graupl-button-border-radius prop to 3rem, the button border radius should be 3rem.

We'd need to add the top/bottom/left/right checks as well, but this was we'd know what's verified to be happening.

Solution

Implement tests