Extend EscapeOutputSniff unit test with Widget code which should pass

For #159
Accept unescaped WP_Widget::get_field_id() and WP_Widget::get_field_name()
Merge pull request #160 from WordPress-Coding-Standards/issue-159-escape-output-sniff-whitelist-widget-methods

Fix: EscapeOutputSniff should not flag echoing WP_Widget::get_field_id() and WP_Widget::get_field_name()