Allows the artificial multiplication of posts in WordPress queries for testing purposes.

pilau-multiply-posts.php

<?php

/**
 * Allow the multiplication of posts in query results for testing purposes.
 * 
 * In the query args, set pilau_multiply to the number you want the posts multiplying by.
 * NOTE: If using get_posts() instead of WP_Query, you will need to set suppress_filters to true.
 */
add_filter( 'the_posts', 'pilau_multiply_posts', 10, 2 );
function pilau_multiply_posts( $posts, $query ) {

	// Is the query set to multiply
	if ( isset( $query->query['pilau_multiply'] ) && $query->query['pilau_multiply'] ) {

		// Store original set of posts
		$posts_original = $posts;

		// Multiply
		for ( $i = 1; $i < $query->query['pilau_multiply']; $i++ ) {
			$posts = array_merge( $posts, $posts_original );
		}

		// Adjust count
		$query->found_posts = count( $posts );

	}

	return $posts;
}