Backport: Theme JSON Tests: Refactor base styles to a static variable

[scruffian]

Backports WordPress/gutenberg#58975 to core.

I haven't yet been able to run the tests to check it works.

Trac ticket:

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[tellthemachines]

I'm not sure there's much benefit to syncing this to core, because these tests really need a wider refactor. I proposed that in WordPress/gutenberg#60842 (feedback welcome!) and intend to work on it in the next weeks.

The fact that this PR already has conflicts after a few days of being open illustrates the problem - every time we make updates to theme.json styles, these test strings also need to be updated, and the updates inevitably conflict 😬

That said, I don't have anything against this going into core, and am happy to help out with reviewing/committing once conflicts are solved!

Edit: if we go forward with this it also needs a trac ticket.

[ellatrix]

Let's keep this for the end, when all the other backports touching these tests are in.

[tellthemachines]

Given #6734 removes base styles from all tests where they aren't relevant, is this change still needed?

[scruffian]

Yeah this isn't needed anymore