[5.x] Addon Testing Changes

[duncanmcclean]

This pull request makes a couple of changes around addon testing:

• Moves the AddonTestCase introduced in [5.x] Simplify TestCase for addons #9573 into the Statamic\Testing namespace
• Makes the PreventSavingStacheItemsToDisk trait available for addons
  • This trait prevents Stache items from being kept around on the disk after the test suite has finished running.
  • I had to make some changes to the trait in order for the paths to be correct. Otherwise, the Stache files would live in vendor/statamic/cms/src/Testing/Concerns/__fixtures__.