Test case discovery is not very intuitive

The `_tc` suffix is in my opinion still a good default when running a whole test suite, but when the user explicitly specifies an entity as toplevel, it should use that regardless of any (default) pattern nonsense in the targets. Patterns were kind of an afterthought anyway, and should be implemented as part of toplevel file discovery vs. filtering toplevels from test cases.

Also annoying in the current discovery system is that there's no include flag for including paths from which only dependencies are loaded. Currently you'd use `-i`, but then any test cases in that path will be made part of the test suite, which is probably not what you want when you're working with someone else's library. Maybe `-l`?

Look at the test case discovery logic in the codebase, likely in a file handling command-line arguments or test runner. Understand how the `_tc` suffix is applied and how patterns filter toplevel entities. Check for existing flags like `-i` and see where test paths are processed. The goal is to modify discovery so an explicitly specified toplevel entity is used directly, and to add a new flag (maybe `-l`) for including paths without adding their test cases.

Written by the indexing model from the issue text.