clientv3/integration: Get with context timeout

Address https://github.com/coreos/etcd/pull/8762#discussion_r147019068.

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

clientv3/integration/server_shutdown_test.go

@@ -48,7 +48,12 @@ func TestBalancerUnderServerShutdownWatch(t *testing.T) {
 	defer watchCli.Close()
 
 	// wait for eps[lead] to be pinned
-	watchCli.Get(context.Background(), "foo")
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	_, err = watchCli.Get(ctx, "foo")
+	cancel()
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	// add all eps to list, so that when the original pined one fails
 	// the client can switch to other available eps