Adding barrier and timeout in failing test

Fixes #2557

Author: arunjose696

tests/org.eclipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/Test_org_eclipse_swt_graphics_GC.java

@@ -30,6 +30,7 @@
 import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
 import java.lang.ref.WeakReference;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 
 import org.eclipse.swt.SWT;
@@ -249,7 +250,7 @@ public void test_dispose() {
 
 
 @Test
-public void test_drawImage_nonAutoScalableGC_bug_2504() {
+public void test_drawImage_nonAutoScalableGC_bug_2504() throws InterruptedException {
 	Shell shell = new Shell(display);
     float targetScale = 2f;
     int srcSize = 50;
@@ -273,15 +274,18 @@ public boolean isAutoScalable() {
     int canvasWidth = Math.round(bounds.width * targetScale);
     int canvasHeight = Math.round(bounds.height * targetScale);
     canvas.setSize(canvasWidth, canvasHeight);
+    AtomicBoolean paintListenerCalled = new AtomicBoolean();
     canvas.addPaintListener(e -> {
         e.gc.drawImage(image, 0, 0, bounds.width, bounds.height,
                        0, 0, canvasWidth, canvasHeight);
+        paintListenerCalled.set(true);
     });
 
     shell.open();
-    while (display.readAndDispatch()) {
-    }
-    Image target = new Image(display, canvasWidth, canvasHeight);
+	int timeoutMillis = 2000;
+	SwtTestUtil.processEvents(timeoutMillis, () -> paintListenerCalled.get());
+	assertTrue(paintListenerCalled.get(), "paint listener was never called");
+	Image target = new Image(display, canvasWidth, canvasHeight);
     GC gcCopy = new GC(canvas);
     gcCopy.copyArea(target, 0, 0);
     gcCopy.dispose();