Run tests without optimization

[RobinMalfait]

This PR improves the tests by not running the optimization step (via Lightning CSS) in places where we know that nothing should be generated.

In fact, if we generate invalid CSS where we expected that nothing was being generated, then Lightning CSS will throw the CSS out. This means that we have a bug somewhere in our code, and we should fix it but we don't know about it.

Bonus points: this improves the performance of the tests a tiny bit.

[RobinMalfait]

• #14704 👈 (View in Graphite)
• #14703
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[RobinMalfait]

Closing this, if we want this we can re-enable it later.