Use newInstanceWithoutConstructor when possible

Comment says

// We have to use this dirty trick instead of ReflectionClass::newInstanceWithoutConstructor()
// because of https://github.com/sebastianbergmann/phpunit-mock-objects/issues/154

As I see the only reason to use unserialize here is internal classes (or not?). So how about using unserialize for internal classes and newInstanceWithoutConstructor for other?