Fix UITestUtil.processEventsUntil causing test failures in RCPTestWorkbenchAdvisor

The previous implementation called UITestUtil.processEventsUntil() which
internally calls PlatformUI.getWorkbench().getDisplay(). However, during
postStartup(), the workbench display may not be reliably accessible via
PlatformUI.getWorkbench(), causing the test to fail.

This change replaces the UITestUtil calls with direct Display.getCurrent()
usage and implements the same event processing logic inline. This ensures
that:
1. We wait for async/sync operations with DisplayAccess to complete
2. We process remaining events to detect any regressions
3. We don't rely on PlatformUI.getWorkbench() during startup

Fixes the following test failures:
- Async run during startup
- Sync from un-qualified thread ran during startup
- Async from un-qualified thread ran during startup

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vogella

tests/org.eclipse.ui.tests.harness/src/org/eclipse/ui/tests/harness/util/RCPTestWorkbenchAdvisor.java

@@ -234,10 +234,29 @@ public void postStartup() {
 		// Pump the event loop to ensure async runnables execute before marking as started
 		// This prevents the original race condition where async variables might not be set yet
 		// Wait until the variables that should be set during startup are actually set to TRUE
-		UITestUtil.processEventsUntil(() -> Boolean.TRUE.equals(syncWithDisplayAccess) && Boolean.TRUE.equals(asyncWithDisplayAccess), 5000);
-		// Process any remaining events to allow variables that should NOT be set during startup
-		// to accidentally execute (to detect regression)
-		UITestUtil.processEvents();
+		Display display = Display.getCurrent();
+		if (display != null) {
+			long startTime = System.currentTimeMillis();
+			while (!(Boolean.TRUE.equals(syncWithDisplayAccess) && Boolean.TRUE.equals(asyncWithDisplayAccess))) {
+				if (System.currentTimeMillis() - startTime > 5000) {
+					System.err.println("WARNING: Timeout waiting for sync/async operations with DisplayAccess");
+					break;
+				}
+				if (!display.readAndDispatch()) {
+					try {
+						Thread.sleep(10);
+					} catch (InterruptedException e) {
+						Thread.currentThread().interrupt();
+						break;
+					}
+				}
+			}
+			// Process any remaining events to allow variables that should NOT be set during startup
+			// to accidentally execute (to detect regression)
+			while (display.readAndDispatch()) {
+				// Process all pending events
+			}
+		}
 
 		synchronized (RCPTestWorkbenchAdvisor.class) {
 			started = true;