command: move func to extract stdout/err in tests

In order to test the output from a test command, the commandOutput
function exists in the command_test.go file.

Since its behaviour is linked to the test meta that we produce in the
test_utils.go file, we move it there, and rename it to
`GetStdoutAndErrFromTestMeta' to make it clearer what to expect from it.

command/command_test.go

@@ -22,13 +22,6 @@ func fatalCommand(t *testing.T, m Meta) {
 		err.String())
 }
 
-func outputCommand(t *testing.T, m Meta) (string, string) {
-	ui := m.Ui.(*packersdk.BasicUi)
-	out := ui.Writer.(*bytes.Buffer)
-	err := ui.ErrorWriter.(*bytes.Buffer)
-	return out.String(), err.String()
-}
-
 func testFixtureContent(n ...string) string {
 	path := filepath.Join(append([]string{fixturesDir}, n...)...)
 	b, err := ioutil.ReadFile(path)

command/test_utils.go

@@ -32,6 +32,14 @@ func TestMetaFile(t *testing.T) Meta {
 	}
 }
 
+// GetStdoutAndErrFromTestMeta extracts stdout/stderr from a Meta created by TestMetaFile
+func GetStdoutAndErrFromTestMeta(t *testing.T, m Meta) (string, string) {
+	ui := m.Ui.(*packersdk.BasicUi)
+	out := ui.Writer.(*bytes.Buffer)
+	err := ui.ErrorWriter.(*bytes.Buffer)
+	return out.String(), err.String()
+}
+
 // testCoreConfigBuilder creates a packer CoreConfig that has a file builder
 // available. This allows us to test a builder that writes files to disk.
 func testCoreConfigBuilder(t *testing.T) *packer.CoreConfig {

command/validate_test.go

@@ -137,7 +137,7 @@ func TestValidateCommandBadVersion(t *testing.T) {
 		t.Errorf("Expected exit code 1")
 	}
 
-	stdout, stderr := outputCommand(t, c.Meta)
+	stdout, stderr := GetStdoutAndErrFromTestMeta(t, c.Meta)
 	expected := `Error: 
 
 This template requires Packer version 101.0.0 or higher; using 100.0.0