Rename go files under command group dir (#245)

- Files under service dir
- Files under revision dir
- Files under plugin dir

[issue 229](https://github.com/knative/client/issues/229)

pkg/kn/commands/service/list_flags_test.go

@@ -25,7 +25,7 @@ import (
 func TestServiceListFlags(t *testing.T) {
 	var cmd *cobra.Command
 
-	t.Run("adds service list flag", func(t *testing.T) {
+	t.Run("verify service list flags", func(t *testing.T) {
 		serviceListFlags := NewServiceListFlags()
 
 		cmd = &cobra.Command{}