[SPARK-23834][test] Wait for connection before disconnect in LauncherServer test.

It was possible that the disconnect() was called on the handle before the
server had received the handshake messages, so no connection was yet
attached to the handle. The fix waits until we're sure the handle has been
mapped to a client connection.

Author: Marcelo Vanzin

launcher/src/test/java/org/apache/spark/launcher/LauncherServerSuite.java

@@ -196,6 +196,14 @@ public void testAppHandleDisconnect() throws Exception {
       Socket s = new Socket(InetAddress.getLoopbackAddress(), server.getPort());
       client = new TestClient(s);
       client.send(new Hello(secret, "1.4.0"));
+      client.send(new SetAppId("someId"));
+
+      // Wait until we know the server has received the messages and matched the handle to the
+      // connection before disconnecting.
+      eventually(Duration.ofSeconds(1), Duration.ofMillis(10), () -> {
+        assertEquals("someId", handle.getAppId());
+      });
+
       handle.disconnect();
       waitForError(client, secret);
     } finally {