Prevent test folder deletion on running `flutter create --empty` on an existing app project

[victoreronmosele]

This PR modifies the flutter create --empty command to not delete the test/ folder when run on an existing app project.

Before:

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:

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

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• [x] I signed the CLA.
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] I followed the breaking change policy and added Data Driven Fixes where supported.
• [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[gspencergoog]

When not creating a new project, won't this recreate test/widget_test.dart if it doesn't exist?

Ideally, this should probably just never write a file to test at all, and skip the delete step.

[andrewkolos]

When not creating a new project, won't this recreate test/widget_test.dart if it doesn't exist?

Indeed, this seems to be the case.

Ideally, this should probably just never write a file to test at all, and skip the delete step.

Agreed.