Fix race condition in RCPTestWorkbenchAdvisor by removing premature event processing

The previous fix added a CountDownLatch to wait for operations with DisplayAccess,
but also included a display.readAndDispatch() loop that processed ALL pending events
in postStartup(). This caused deferred operations (without DisplayAccess) to execute
before startup completed, violating the contract that such operations should be
deferred until after startup.

This change removes the display.readAndDispatch() loop while keeping the latch-based
synchronization for operations with DisplayAccess. The test expects:
- Operations WITH DisplayAccess → run during startup
- Operations WITHOUT DisplayAccess → deferred until after startup
- Direct asyncExec from UI thread → deferred until after 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

@@ -232,16 +232,6 @@ public void postStartup() {
 			}
 		}
 
-		// Process remaining events to allow any pending runnables to execute
-		// This gives operations WITHOUT DisplayAccess a chance to run if there are bugs
-		Display display = Display.getCurrent();
-		if (display != null) {
-			// Process all pending events
-			while (display.readAndDispatch()) {
-				// Keep processing
-			}
-		}
-
 		// Now mark as started - operations with DisplayAccess should have completed
 		// Operations without DisplayAccess should still be pending (deferred)
 		synchronized (RCPTestWorkbenchAdvisor.class) {