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.
fixed compatibility issue with PHP 5.3.3

is_a in PHP 5.3.3 does not support third parameter (allow_string) thus $object parameter cannot be string.
+++ test of test case static property preservation
typo fix