Refactor test to make it more robust.

The test was failing for NIO2 after changes to
TesterFirehoseServer.SEND_TIME_OUT_MILLIS. This fixes that failure and
makes the test more robust for any future changes.

test/org/apache/tomcat/websocket/server/TestSlowClient.java

@@ -64,17 +64,24 @@ public void testSendingFromAppThread() throws Exception {
         // Trigger the sending of the messages from the server
         wsSession.getBasicRemote().sendText("start");
 
-        // Wait for server to close connection (it shouldn't)
-        // 20s should be long enough even for the slowest CI system. May need to
-        // extend this if not.
+        /*
+         * Wait for server to experience a write timeout. This should take TesterFirehoseServer.SEND_TIME_OUT_MILLIS
+         * plus however long it takes for the network buffer to fill up which should be a few milliseconds. An
+         * additional 10s should be enough even for the slowest CI system.
+         *
+         * As soon as the server has experienced the write timeout, send the session close message from the client and
+         * close the network connection.
+         */
+        Thread.sleep(TesterFirehoseServer.SEND_TIME_OUT_MILLIS);
         int count = 0;
-        while (wsSession.isOpen() && count < 200) {
-            Thread.sleep(100);
+        while (wsSession.isOpen() && TesterFirehoseServer.Endpoint.getErrorCount() == 0 && count < 200) {
+            Thread.sleep(50);
             count++;
         }
+        Assert.assertTrue(TesterFirehoseServer.Endpoint.getErrorCount() > 0);
         Assert.assertTrue(wsSession.isOpen());
 
-        // Cast so we can force the session to be closed quickly.
+        // Cast so we can force the session and the socket to be closed quickly.
         CloseReason cr = new CloseReason(CloseCodes.CLOSED_ABNORMALLY, "");
         ((WsSession) wsSession).doClose(cr, cr, true);