Updated potentialStarter test which covers now also the extensionElement

Test was updated in order to cover:
```
 <extensionElements>
			<activiti:potentialStarter>
				<resourceAssignmentExpression>
					<formalExpression>group2, group(group3), user(user3)</formalExpression>
				</resourceAssignmentExpression>
			</activiti:potentialStarter>
	 </extensionElements>
</xml>
```
More details : http://forums.activiti.org/content/process-initiation-authorization?page=0