This PR modifies the `flutter create --empty` command to not delete the `test/` folder when run on an existing app project.
Before:
```bash
flutter create my_app --empty
mkdir my_app/test
if test -d my_app/test; then echo "test exists"; else echo "test does not exist"; fi # test exists
flutter create my_app --empty
if test -d my_app/test; then echo "test exists"; else echo "test does not exist"; fi # test does not exist
```
After:
```bash
flutter create my_app --empty
mkdir my_app/test
if test -d my_app/test; then echo "test exists"; else echo "test does not exist"; fi # test exists
flutter create my_app --empty
if test -d my_app/test; then echo "test exists"; else echo "test does not exist"; fi # test exists
```
Fixes https://github.com/flutter/flutter/issues/134928