Fix: serialization of test suits static properties

When --backupStaticAttributes switch is set, static properties of classes derived from PHPUnit_Framework_Test are also backed up due to wrong use of instanceof.

<string> instanceof <object>|<class>|<string> is always false. One should use is_a(<stringClassName>, <stringClassName>, true).

This bug affect all versions of PHPUnit that uses PHPUnit_Util_GlobalState class.