bpo-26901: Fix the Argument Clinic test suite (GH-8879)

* Fix Tools/clinic/clinic_test.py: add missing
  FakeClinic.destination_buffers attribute and pass a file argument
  to Clinic().
* Rename Tools/clinic/clinic_test.py to Lib/test/test_clinic.py:
  add temporary Tools/clinic/ to sys.path to import the clinic
  module.

Co-Authored-By: Pablo Galindo <pablogsal@gmail.com>