clientv3/integration: increase timeout in watch connection close tests

Integration tests have dial timeout 5-sec, and it's possible that
balancer retry logic waits 5-sec and test times out because gRPC calls
grpc.downErr function after connection wait starts in retrial part.
Just increasing time-out should be ok. In most cases, grpc.downErr gets
called before starting the wait.

e.g.

=== RUN   TestWatchErrConnClosed
INFO: 2017/10/18 23:55:39 clientv3/balancer: pin "localhost:91847156765553894590"
INFO: 2017/10/18 23:55:39 clientv3/retry: wait 5s for healthy endpoint
INFO: 2017/10/18 23:55:39 clientv3/balancer: unpin "localhost:91847156765553894590" ("grpc: the client connection is closing")
INFO: 2017/10/18 23:55:39 clientv3/health-balancer: "localhost:91847156765553894590" becomes unhealthy ("grpc: the client connection is closing")
--- FAIL: TestWatchErrConnClosed (3.07s)
        watch_test.go:682: wc.Watch took too long

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

clientv3/integration/watch_test.go

@@ -678,7 +678,7 @@ func TestWatchErrConnClosed(t *testing.T) {
 	clus.TakeClient(0)
 
 	select {
-	case <-time.After(3 * time.Second):
+	case <-time.After(10 * time.Second):
 		t.Fatal("wc.Watch took too long")
 	case <-donec:
 	}
@@ -705,7 +705,7 @@ func TestWatchAfterClose(t *testing.T) {
 		close(donec)
 	}()
 	select {
-	case <-time.After(3 * time.Second):
+	case <-time.After(10 * time.Second):
 		t.Fatal("wc.Watch took too long")
 	case <-donec:
 	}