Reusable blocks: Add tests for recursion, subsequent instances

[Bernhard Reiter]

As of Gutenberg 9.8.1, it's possible to insert a reusable block into itself. This creates an infinite recursion, ​triggering a server error (500), and possibly crashing the editor. (We seem to encounter the latter behavior more frequently, per the recent [reusable blocks refactor]​https://github.com/WordPress/gutenberg/pull/27887.)

This has been recently fixed in Gutenberg (​gb#28405 and ​gb#28428). (It was then amended by ​gb#28461 to continue to allow rendering of multiple non-nested reusable blocks in a post.)

Reusable blocks are covered by PHP unit tests found in ​`tests/phpunit/tests/blocks/render-reusable.php`.

We should add tests to:

• Ensure recursions are prevented, and that
• Rendering of subsequent (non-nested) reusable blocks still works.

I have a diff ready that I'll file as a PR against the wordpress-develop repo shortly.

(On a sidenote, in order to prevent regressions in the future, we should consider running Core's unit tests against PRs in the Gutenberg repo, see ​gb#26418).

As of Gutenberg 9.8.1, it's possible to insert a reusable block into itself. This creates an infinite recursion, ​triggering a server error (500), and possibly crashing the editor. (We seem to encounter the latter behavior more frequently, per the recent ​reusable blocks refactor.)

This has been recently fixed by @mcsf in Gutenberg (​https://github.com/WordPress/gutenberg/pull/28405 and ​https://github.com/WordPress/gutenberg/pull/28428). (It was then amended by ​https://github.com/WordPress/gutenberg/pull/28461 to continue to allow rendering of multiple non-nested reusable blocks in a post.)

Reusable blocks are covered by PHP unit tests found in ​`tests/phpunit/tests/blocks/render-reusable.php`.

This PR adds tests to:

• Ensure recursions are prevented, and that
• Rendering of subsequent (non-nested) reusable blocks still works.

Please note that these newly added tests will currently fail (e.g. in CI), unless the aforementioned Gutenberg PRs are carried over to WordPress**

(On a sidenote, in order to prevent regressions in the future, we should consider running Core's unit tests against PRs in the Gutenberg repo, see ​https://github.com/WordPress/gutenberg/issues/26418).

Trac ticket: https://core.trac.wordpress.org/ticket/52364

cc/ @mcsf @gziolo @sirreal

[peterwilsoncc]

In 52364.diff​ :

• bring the PR patch in to trac
• As many globals are reset between tests, I've called $block_type->render() twice in test_render_subsequent() to make sure any relevant globals haven't been reset since the previous test.
• All post objects are reset to their original state at the end of each test, I've removed the manual call to wp_update_post() at the end of test_recursive_render_warning() as adds unnecessary database writes to the test suite.

This looks ready to me, marking as such.

Related #51802.

[peterwilsoncc]

In 50382:

Editor: Additional tests for reusable blocks.

Ensure that subsequent renders of a reusable block will render correctly and that recursively inserting a reusable block into itself does not cause an internal server (500) error.

Props bernhard-reiter, SergeyBiryukov.
Fixes #52364.

Closed with changeset https://core.trac.wordpress.org/changeset/50382