title: AttributeError: type object 'NoDbTestRunner' has no attribute 'split'
body: <p>I'm trying to create a DiscoverRunner for Django 2.0 and python3 to skip DB creation, but get this error:</p>
  
  <pre><code>File "manage.py", line 15, in &lt;module&gt;
  execute_from_command_line(sys.argv)
  File "/home/mlmireles/.virtualenvs/risk-analysis/lib/python3.6/site-packages/django/core/management/__init__.py", line 371, in execute_from_command_line
  utility.execute()
  File "/home/mlmireles/.virtualenvs/risk-analysis/lib/python3.6/site-packages/django/core/management/__init__.py", line 365, in execute
  self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/mlmireles/.virtualenvs/risk-analysis/lib/python3.6/site-packages/django/core/management/commands/test.py", line 26, in run_from_argv
  super().run_from_argv(argv)
  File "/home/mlmireles/.virtualenvs/risk-analysis/lib/python3.6/site-packages/django/core/management/base.py", line 280, in run_from_argv
  parser = self.create_parser(argv[0], argv[1])
  File "/home/mlmireles/.virtualenvs/risk-analysis/lib/python3.6/site-packages/django/core/management/base.py", line 254, in create_parser
  self.add_arguments(parser)
  File "/home/mlmireles/.virtualenvs/risk-analysis/lib/python3.6/site-packages/django/core/management/commands/test.py", line 47, in add_arguments
  test_runner_class = get_runner(settings, self.test_runner)
  File "/home/mlmireles/.virtualenvs/risk-analysis/lib/python3.6/site-packages/django/test/utils.py", line 306, in get_runner
  test_path = test_runner_class.split('.')
  AttributeError: type object 'NoDbTestRunner' has no attribute 'split'
  </code></pre>
  
  <p>Here's the code for the runner:</p>
  
  <pre><code>from django.test.runner import DiscoverRunner
  
  class NoDBTestRunner(DiscoverRunner):
      def setup_databases(self, **kwargs):
          pass
  
      def teardown_databases(self, *args, **kwargs):
          pass
  </code></pre>