This is a child issue of #3057420: [meta] How to deprecate Simpletest with minimal disruption
Problem/Motivation
We want to decouple the test running infrastructure from the simpletest module itself.
Many tests use Drupal\Core\StreamWrapper\PublicStream\TestFileCreationTrait::getTestFiles() to generate temporary fixture files.
TestFileCreationTrait::getTestFiles() uses some files within the simpletest module as fixtures.
We can easily see this in #3075490-11: Move simpletest module to contrib where we experimentally remove the simpletest module and then run a druaplci test build. Most of the test fails are related to TestFileCreationTrait.
Proposed resolution
Move the fixture files to a new location.
Change Drupal\Core\StreamWrapper\PublicStream\TestFileCreationTrait::getTestFiles() to use the new location.