Default to sorting user's test cases for now

Goals:

1. Default to :random for newly generated applications
2. Default to :sorted for existing applications with a warning
3. Only show the warning once
4. Only show the warning if the app actually uses AS::TestCase

Fixes #16769
Merge pull request #16839 from chancancode/default_test_order

Default to sorting user's test cases for now
Fix typo
Set the test order to be random in the generated app

This will avoid the warning in our tests. We delete all the
config/environments/ files in the test app so our default code will not
work.