Do not allow test files to import files that have their own test cases

When parallelizing test runs based on file names, some test cases execute in multiple shards because they get imported by other files. This issue tracks the effort to disallow the import of test cases from other test cases (example of such a situation in the first mention of this issue)

Start with the example in the first mention of this issue and trace how test files import one another and how file-name-based sharding discovers tests. Identify the test-runner or validation entry point involved; done means imports of test cases from other test files are rejected so cases are not duplicated across shards.

Written by the indexing model from the issue text.