Add PHP unit test coverage for duotone block supports

[getdave]

As evidenced by this PR
the PHP side of the duotone block supports is potentially brittle and prone to regressions.

We should break gutenberg_render_duotone_support into smaller units and add tests for the results to provide more confidence. This should also make the code easier to reason about.

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@draganescu @scruffian Thanks for catching custom color breakage. I've reinstated some code I accidentally removed during a refactor. Points to this code being brittle and requiring tests.

Originally posted by @getdave in #48318 (comment)

[getdave]

Related #48347

[im3dabasia]

Hi @getdave,

Are we still moving forward with this? If so, I’d love to contribute and help out.

I’m planning to add some tests for the duotone-picker, as mentioned in this PR #48353 (comment)