Fix race condition in serverStream_triggerEvent_afterClose test.

Wait for the server stream to be fully closed (via awaitClose) before
calling triggerEvent, to ensure the transport has processed the
cancellation and marked the listener as closed. This fixes flakiness in
slower transports like Jetty.

TAG=agy
CONV=e1bfa5a2-e855-4f79-abdd-ef2b264977be

Author: kannanjgithub

core/src/testFixtures/java/io/grpc/internal/AbstractTransportTest.java

@@ -2139,6 +2139,8 @@ public void serverStream_triggerEvent_afterClose() throws Exception {
     // Close the stream from client side
     clientStream.cancel(Status.CANCELLED);
 
+    serverStreamListener.awaitClose(TIMEOUT_MS, TimeUnit.MILLISECONDS);
+
     Object event = new Object();
     serverStream.triggerEvent(event);