Layout: backport fix to ensure blocks without layout support do not receive layout classnames

[andrewserong]

Backport changes from the Gutenberg plugin in: WordPress/gutenberg#56187

Prevent layout classnames from being output on blocks where there is no layout support and child layout classnames are not being output. An example is a paragraph that is a child of a group block, and that has selfStretch set to fit. In this case, no layout classnames should be added to the paragraph block, but prior to WordPress/gutenberg#56187 they were being added unexpectedly.

This PR also adds a test to cover the fix. For further context and sample test markup, see the PR description of WordPress/gutenberg#56187

Trac ticket: https://core.trac.wordpress.org/ticket/60292

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[github-actions]

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[tellthemachines]

Change LGTM and works as expected. Thanks for adding the test!

[tellthemachines]

Committed in r57328.

[andrewserong]

Thanks for reviewing and committing!