HTML API: Test cleanup

[sirreal]

Trac ticket: Core-59647

Rename $p variable to $processor in tests for clarity (#5975 (comment)).

Use static data providers. A mix of static and non-static data providers were used in HTML API tests.
Data providers are required to be static in the next PHPUnit version and there's no harm in using
them consistently now.

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[github-actions]

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[dmsnell]

Lovely. Thank you, sir! for real!

[dmsnell]

Merged in [57508]
f16c792