Wait for standby to have a working grpc connection before we try to use it (#16905)

Also teach WaitForStandbyNode to do a better job waiting for standbys to be healthy.

helper/testhelpers/testhelpers.go

@@ -349,6 +349,9 @@ func WaitForStandbyNode(t testing.T, core *vault.TestClusterCore) {
 		if isLeader, _, clusterAddr, _ := core.Core.Leader(); isLeader != true && clusterAddr != "" {
 			return
 		}
+		if core.Core.ActiveNodeReplicationState() == 0 {
+			return
+		}
 
 		time.Sleep(time.Second)
 	}