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

[andrewserong]

This ticket is to track backporting a fix from the Gutenberg repo, which landed in ​https://github.com/WordPress/gutenberg/pull/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 ​https://github.com/WordPress/gutenberg/pull/56187 they were being added unexpectedly.

Backport changes from the Gutenberg plugin in: ​https://github.com/WordPress/gutenberg/pull/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 ​https://github.com/WordPress/gutenberg/pull/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 ​https://github.com/WordPress/gutenberg/pull/56187

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

[isabel_brison]

In 57328:

Editor: fix classname output on blocks without layout.

Prevents layout classnames from being output on blocks with no layout support and no child layout classnames by returning early from wp_render_layout_support_flag.

Props andrewserong.
Fixes #60292.

Committed in r57328.

Thanks for reviewing and committing!