test / pkg_resources crashes on native (__init__.py-less) namespace package

setup.py test fails with a confusing exception if the test suite is a "py3 native" namespace package (no __init__.py).

Reproducer

setup.py:

from setuptools import setup

setup(name='reproducer',
      version='0.0',
      test_suite='tests',
)

tests is an empty directory (it might have some tests, but no __init__.py)

Run python setup.py test

Result

Traceback (most recent call last):
  File ".../setup.py", line 3, in <module>
    setup(name='reproducer',
  ...
  File ".../unittest/loader.py", line 220, in loadTestsFromNames
    suites = [self.loadTestsFromName(name, module) for name in names]
  ...
  File ".../setuptools/command/test.py", line 47, in loadTestsFromModule
    for file in resource_listdir(module.__name__, ''):
  File ".../pkg_resources/__init__.py", line 1162, in resource_listdir
    return get_provider(package_or_requirement).resource_listdir(
  File ".../pkg_resources/__init__.py", line 364, in get_provider
    return _find_adapter(_provider_factories, loader)(module)
  File ".../site-packages/pkg_resources/__init__.py", line 1392, in __init__
    self.module_path = os.path.dirname(getattr(module, '__file__', ''))
  File ".../posixpath.py", line 152, in dirname
    p = os.fspath(p)
TypeError: expected str, bytes or os.PathLike object, not NoneType

I know pkg_resources is being deprecated in favor of importlib.resources, but ... this is setuptools itself using it.
I'm also aware there are better alternatives to setup.py test (but if the official position is "don't use it", I'd welcome a statement from setuptools maintainers saying that, so I can link people to it).

Filing this mostly to let you know. I don't really mind if this isn't fixed.

Reproduce the failure with the shown setup.py and empty tests directory using python setup.py test. Read setuptools/command/test.py and pkg_resources/init.py at the traceback locations, then verify that testing a native namespace package no longer crashes.