[classlib][test] isHarmony method in the swing tests

[Mikhail Loenko]

Did I understand correctly that it's a temporary solution to
differentiate between
"api" and "impl" tests?

package javax.swing.text;
<...>
public class PlainViewI18N_LineViewTest extends SwingTestCase {
<...>
   public void testGetPreferredSpan01() throws Exception {
       if (!isHarmony()) {
           return;
       }