Skip icon test in headless mode.

property-loader/src/test/java/com/github/weisj/darklaf/icons/IconUtilTest.java

@@ -36,6 +36,7 @@ public class IconUtilTest {
 
     @Test
     public void testWeakFrameCache() {
+        if (GraphicsEnvironment.isHeadless()) return;
         Object obj = new Object();
         WeakReference<Object> ref = new WeakReference<>(obj);
         int count = 100;