linkage_checker: exclude test deps from being considered runtime dep

[cho-m]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

A couple situations this helps:

• Avoid no linkage message for test dependencies, e.g. harfbuzz:

  Dependencies with no linkage:
    pygobject3
  

• Properly detect when a :test-dependency that is also an indirect dep should actually be made a direct runtime dependency.