Fixed a bug that HttpServer in unit test does not start correctly. (#2361)

Because the http client in unit test try to connect to the server when
server is not ready yet.

fe/src/test/java/org/apache/doris/http/DorisHttpTestCase.java

@@ -255,7 +255,9 @@ abstract public class DorisHttpTestCase {
         httpServer.setup();
         httpServer.start();
         // must ensure the http server started before any unit test
-        Thread.sleep(5000);
+        while (!httpServer.isStarted()) {
+            Thread.sleep(500);
+        }
         doSetUp();
     }