Backport https://github.com/WordPress/gutenberg/pull/55354 to Core

[anton-vlasenko]

This PR aims to backport WordPress/gutenberg#55354 to Core to ensure that Tests_Theme_wpThemeJson::test_set_spacing_sizes_should_detect_invalid_spacing_scale is compatible with PHPUnit 10

Trac ticket: https://core.trac.wordpress.org/ticket/60305

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[youknowriad]

Committed in https://core.trac.wordpress.org/changeset/57332