Add infrastructure for testing cli.Command.Action handlers

Also split creation of cli.App from main(), and add a test helper
function.

This does not change behavior at the moment, but will allow writing
tests of the command handlers.

cmd/skopeo/main.go

@@ -17,7 +17,8 @@ const (
 	usage = `interact with registries`
 )
 
-func main() {
+// createApp returns a cli.App to be run or tested.
+func createApp() *cli.App {
 	app := cli.NewApp()
 	app.Name = "skopeo"
 	if gitCommit != "" {
@@ -67,6 +68,11 @@ func main() {
 		standaloneSignCmd,
 		standaloneVerifyCmd,
 	}
+	return app
+}
+
+func main() {
+	app := createApp()
 	if err := app.Run(os.Args); err != nil {
 		logrus.Fatal(err)
 	}

cmd/skopeo/main_test.go

@@ -0,0 +1,14 @@
+package main
+
+import "bytes"
+
+// runSkopeo creates an app object and runs it with args, with an implied first "skopeo".
+// Returns output intended for stdout and the returned error, if any.
+func runSkopeo(args ...string) (string, error) {
+	app := createApp()
+	stdout := bytes.Buffer{}
+	app.Writer = &stdout
+	args = append([]string{"skopeo"}, args...)
+	err := app.Run(args)
+	return stdout.String(), err
+}