Add test for migration output

Previously printing referenced `os.Stderr` instead of `Err` as done elsewhere
Having fixed this, it seems fit to verify migration printing output

Author: jdsutherland

cmd/submit.go

@@ -134,7 +134,7 @@ func runSubmit(cfg config.Config, flags *pflag.FlagSet, args []string) error {
 		return err
 	}
 	if verbose, _ := flags.GetBool("verbose"); verbose {
-		fmt.Fprintf(os.Stderr, migrationStatus.String())
+		fmt.Fprintf(Err, migrationStatus.String())
 	}
 	metadata, err := workspace.NewExerciseMetadata(exerciseDir)
 	if err != nil {

cmd/submit_test.go

@@ -1,6 +1,7 @@
 package cmd
 
 import (
+	"bytes"
 	"encoding/json"
 	"io/ioutil"
 	"net/http"
@@ -187,15 +188,12 @@ func TestSubmitFiles(t *testing.T) {
 }
 
 func TestLegacyMetadataMigration(t *testing.T) {
-	oldOut := Out
 	oldErr := Err
-	Out = ioutil.Discard
-	Err = ioutil.Discard
 	defer func() {
-		Out = oldOut
 		Err = oldErr
 	}()
-	// The fake endpoint will populate this when it receives the call from the command.
+	Err = &bytes.Buffer{}
+
 	submittedFiles := map[string]string{}
 	ts := fakeSubmitServer(t, submittedFiles)
 	defer ts.Close()
@@ -239,14 +237,18 @@ func TestLegacyMetadataMigration(t *testing.T) {
 	ok, _ = exercise.HasMetadata()
 	assert.False(t, ok)
 
-	err = runSubmit(cfg, pflag.NewFlagSet("fake", pflag.PanicOnError), []string{file})
+	flags := pflag.NewFlagSet("fake", pflag.PanicOnError)
+	flags.Bool("verbose", true, "")
+
+	err = runSubmit(cfg, flags, []string{file})
 	assert.NoError(t, err)
 	assert.Equal(t, "This is a file.", submittedFiles["file.txt"])
 
 	ok, _ = exercise.HasLegacyMetadata()
 	assert.False(t, ok)
 	ok, _ = exercise.HasMetadata()
 	assert.True(t, ok)
+	assert.Regexp(t, "Migrated metadata", Err)
 }
 
 func TestSubmitWithEmptyFile(t *testing.T) {