using discover with testrepository is not obvious (needs --load-list)

I would like to have a .testr.conf like this::

  test_command=python -m subunit.run discover $IDLIST

(subunit uses testtools's TestProgram)

This works for running the whole suite, but when I want to run a subset of the test suite, I cannot. I get this error:

$ python -m testtools.run discover devportalbinary.tests.test_binary devportalbinary.tests.test_binary_backend
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/usr/lib/python2.7/dist-packages/testtools/run.py", line 332, in <module>
    main(sys.argv, sys.stdout)
  File "/usr/lib/python2.7/dist-packages/testtools/run.py", line 329, in main
    program = TestProgram(argv=argv, testRunner=runner, stdout=stdout)
  File "/usr/lib/python2.7/dist-packages/testtools/run.py", line 155, in __init__
    self.parseArgs(argv)
  File "/usr/lib/python2.7/dist-packages/testtools/run.py", line 194, in parseArgs
    self._do_discovery(argv[2:])
  File "/usr/lib/python2.7/dist-packages/testtools/run.py", line 303, in _do_discovery
    self.test = loader.discover(start_dir, pattern, top_level_dir)
  File "/usr/lib/python2.7/unittest/loader.py", line 202, in discover
    raise ImportError('Start directory is not importable: %r' % start_dir)
ImportError: Start directory is not importable: 'devportalbinary.tests.test_binary'

Not sure if this is really a testrepository bug.