Add some unit tests to the node and swarm cli code

Start work on adding unit tests to our cli code in order to have to
write less costly integration test.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

command/task/print.go

@@ -61,7 +61,7 @@ func (t tasksBySlot) Less(i, j int) bool {
 // Print task information in a table format.
 // Besides this, command `docker node ps <node>`
 // and `docker stack ps` will call this, too.
-func Print(dockerCli *command.DockerCli, ctx context.Context, tasks []swarm.Task, resolver *idresolver.IDResolver, noTrunc bool) error {
+func Print(dockerCli command.Cli, ctx context.Context, tasks []swarm.Task, resolver *idresolver.IDResolver, noTrunc bool) error {
 	sort.Stable(tasksBySlot(tasks))
 
 	writer := tabwriter.NewWriter(dockerCli.Out(), 0, 4, 2, ' ', 0)
@@ -74,7 +74,7 @@ func Print(dockerCli *command.DockerCli, ctx context.Context, tasks []swarm.Task
 }
 
 // PrintQuiet shows task list in a quiet way.
-func PrintQuiet(dockerCli *command.DockerCli, tasks []swarm.Task) error {
+func PrintQuiet(dockerCli command.Cli, tasks []swarm.Task) error {
 	sort.Stable(tasksBySlot(tasks))
 
 	out := dockerCli.Out()