Update bootstrap.php so the tests can run on both HHVM and Zend

PDOStatement does not have a direct constructor (i.e., __construct). Newing up a PDOStatement or subclass (e.g., MockPDOStatement) does technically work in Zend, but HHVM throws. A correct way to have the code currently structured in bootstrap.php work correctly on both Zend and HHVM is provide a default constructor and execute method in MockPDOStatement. It would probably best long term to avoid newing up a PDOStatement and create the queries another way. But this works for now.