Fix race condition in test.

Make onInitialized mechanism in SimpleLanguageServer more 'robust' w.r.t. to
subtle timing issues.

The intention of onInitalize is typically to guarantee some initialization code.
Therefore it should also be called even if the 'initialized' even is already past.

headless-services/commons/commons-language-server/src/main/java/org/springframework/ide/vscode/commons/languageserver/util/SimpleLanguageServer.java

@@ -83,6 +83,7 @@ import reactor.core.Disposable;
 import reactor.core.publisher.Mono;
 import reactor.core.scheduler.Scheduler;
 import reactor.core.scheduler.Schedulers;
+import static org.springframework.ide.vscode.commons.languageserver.util.AsyncRunner.*;
 
 /**
  * Abstract base class to implement LanguageServer. Bits and pieces copied from
@@ -131,8 +132,7 @@ public class SimpleLanguageServer implements Sts4LanguageServer, LanguageClientA
 	private boolean hasFileWatcherRegistrationSupport;
 
 	private Consumer<InitializeParams> initializeHandler;
-
-	private Runnable initializedHandler;
+	private CompletableFuture<Void> initialized = new CompletableFuture<Void>();
 
 	private Runnable shutdownHandler;
 
@@ -262,10 +262,7 @@ public class SimpleLanguageServer implements Sts4LanguageServer, LanguageClientA
 		Registration registration = new Registration(WORKSPACE_FOLDERS_CAPABILITY_ID, WORKSPACE_FOLDERS_CAPABILITY_NAME, null);
 		RegistrationParams registrationParams = new RegistrationParams(Collections.singletonList(registration));
 		getClient().registerCapability(registrationParams);
-		Runnable h = this.initializedHandler;
-		if (h!=null) {
-			h.run();
-		}
+		this.initialized.complete(null); // triggers onInitialized handlers.
 	  });
 	}
 
@@ -644,15 +641,7 @@ public class SimpleLanguageServer implements Sts4LanguageServer, LanguageClientA
 	}
 
 	public synchronized void onInitialized(Runnable handler) {
-		if (this.initializedHandler==null) {
-			this.initializedHandler = handler;
-		} else {
-			Runnable oldHandler = this.initializedHandler;
-			this.initializedHandler = () -> {
-				oldHandler.run();
-				handler.run();
-			};
-		}
+		thenLog(log, this.initialized.thenAccept((whocares) -> handler.run()));
 	}
 
 	public synchronized void onShutdown(Runnable handler) {