HDFS-13964. RBF: TestRouterWebHDFSContractAppend fails with No Active Namenode under nameservice. Contributed by Ayush Saxena.

(cherry picked from commit 3bfff0f79e7246cbb2989c720c14cfa2566a9d1a)

hadoop-hdfs-project/hadoop-hdfs-rbf/src/test/java/org/apache/hadoop/fs/contract/router/web/RouterWebHDFSContract.java

@@ -82,6 +82,7 @@ public class RouterWebHDFSContract extends HDFSContract {
           cluster.switchToStandby(ns, NAMENODES[1]);
         }
       }
+      cluster.waitActiveNamespaces();
     } catch (Exception e) {
       cluster = null;
       throw new IOException("Cannot start federated cluster", e);