Fix race condition in WorkbenchPluginTest #1891

Test test case testGetImageRegistryFromAdditionalDisplay() in
org.eclipse.ui.tests.api.WorkbenchPluginTest randomly fails because of a
race condition. A display is initialized in another thread, but the
initialization logic may access the value before it has actually been
set by the other thread. This is fixed by reordering the instructions.

Fixes #1891

tests/org.eclipse.ui.tests/Eclipse UI Tests/org/eclipse/ui/tests/api/WorkbenchPluginTest.java

@@ -73,8 +73,8 @@ private Display initializeDisplayInOtherThread() {
 		TestBarrier2 displayCreationBarrier = new TestBarrier2();
 		new Thread(() -> {
 			Display display = new Display();
-			displayCreationBarrier.setStatus(TestBarrier2.STATUS_DONE);
 			displayReference.set(display);
+			displayCreationBarrier.setStatus(TestBarrier2.STATUS_DONE);
 			while (!display.isDisposed()) {
 				if (!display.readAndDispatch()) {
 					display.sleep();