Test cache is primed rather than function call.

tests/phpunit/tests/rest-api/rest-posts-controller.php

@@ -1706,16 +1706,14 @@ public function test_get_items_primes_parent_post_caches() {
 		wp_cache_delete_multiple( $parent_ids, 'posts' );
 		wp_cache_delete_multiple( $attachment_ids, 'posts' );
 
-		$filter = new MockAction();
-		add_filter( 'update_post_metadata_cache', array( $filter, 'filter' ), 10, 2 );
-
 		$request = new WP_REST_Request( 'GET', '/wp/v2/media' );
 		rest_get_server()->dispatch( $request );
 
-		$args = $filter->get_args();
-		$last = end( $args );
-		$this->assertIsArray( $last, 'The last value is not an array' );
-		$this->assertSameSets( $parent_ids, $last[1] );
+		$this->assertNotFalse( wp_cache_get( $parent_id1, 'posts' ), 'Parent post 1 is expected to be primed.' );
+		$this->assertNotFalse( wp_cache_get( $parent_id1, 'post_meta' ), 'Parent post 1 meta data is expected to be primed.' );
+
+		$this->assertNotFalse( wp_cache_get( $parent_id2, 'posts' ), 'Parent post 2 is expected to be primed.' );
+		$this->assertNotFalse( wp_cache_get( $parent_id2, 'post_meta' ), 'Parent post 2 meta data is expected to be primed.' );
 	}
 
 	public function test_get_items_pagination_headers() {