Fix test workflow issues with WP_Mock and PHP 7.0 compatibility

tests/bootstrap.php

@@ -8,6 +8,10 @@
 // First, we need to load the composer autoloader so we can use WP Mock.
 require_once dirname(__DIR__) . '/vendor/autoload.php';
 
+// Import WP_Mock class
+use WP_Mock\Tools\TestCase;
+use WP_Mock;
+
 // Now call the bootstrap method of WP Mock.
 WP_Mock::bootstrap();