Block Hooks: Add end-to-end (e2e) test coverage.

[ockham]

As @gziolo said in #68926 (review), it is becoming more tedious to test Block Hooks manually, specifically that hooked blocks are inserted in different contexts. Additionally, there are plenty of edge cases, e.g. insertion as first or last child into special blocks like the Post Content, Synced Pattern, or Navigation block. These cases are covered by testing instructions in #67272 (Post Content), #68058 (Synced Pattern), and #57754 (Navigation), respectively, but they involve a lot of manual work.

Instead, we should write e2e tests to cover those cases.

[ockham]

We can probably draw inspiration from the test plugins used in the testing instructions of #67272 and #68058, and maybe generalize them a bit further. E.g. something like

defined( 'ABSPATH' ) || exit;

function insert_hooked_blocks( $hooked_blocks, $position, $anchor_block, $context ) {
	if ( ! $context instanceof WP_Post ) {
		return $hooked_blocks;
	}

	if (
		( $anchor_block === 'core/heading' && $position === 'before' ) ||
		( $anchor_block === 'core/post-content' && $position === 'last_child' ) ||
		( $anchor_block === 'core/block' && $position === 'first_child' )
	) {
		$hooked_blocks[] = 'core/paragraph';
	}

	return $hooked_blocks;
}
add_filter( 'hooked_block_types', 'insert_hooked_blocks', 10, 4 );

function set_hooked_block_inner_html( $hooked_block, $hooked_block_type, $relative_position, $anchor_block ) {
	if (
		( $anchor_block['blockName'] === 'core/heading' && 'before' === $relative_position ) ||
		( $anchor_block['blockName'] === 'core/post-content' && 'last_child' === $relative_position ) ||
		( $anchor_block['blockName'] === 'core/block' && 'first_child' === $relative_position ) ||
	) {
                $hooked_block['innerContent'] = array(
                        sprintf(
                                '<p %s>Hooked block successfully inserted in %s position next to `%s` anchor block.</p>' ),
                                /* block wrapper attrs */,
                                $relative_position,
                                $anchor_block
                        );
                )
	}

	return $hooked_block;
}
add_filter( 'hooked_block_core/paragraph', 'set_hooked_block_inner_html', 10, 4 );

Instead of using a Paragraph block, we could also write a little Block Hooks Debug block that displays the relevant information (relative position and anchor).