old_contents: from test_collectorschannel import *
  from test_collectorstimed import *
  from test_files import *
  from test_importers import *
new_contents: from test_collectorschannel import *
  from test_collectorstimed import *
  from test_files import *
subject: Remove test_importers from plumbing/tests/units/__init__.py (close #23).