Fix PytestCollectionWarning for TestRunner class (#1259)

Fix this warning seen in the pytest output:

```
pytest_django/runner.py:6
  /.../pytest-django/pytest_django/runner.py:6: PytestCollectionWarning: cannot collect test class 'TestRunner' because it has a __init__ constructor (from: tests/test_runner.py)
    class TestRunner:
```

The fix uses [`__test__ = False`](https://docs.pytest.org/en/stable/example/pythoncollection.html#:~:text=boolean%20%5F%5Ftest%5F%5F%20attribute) to flag the class as non-test.

Author: adamchainz

pytest_django/runner.py

@@ -6,6 +6,8 @@
 class TestRunner:
     """A Django test runner which uses pytest to discover and run tests when using `manage.py test`."""
 
+    __test__ = False
+
     def __init__(
         self,
         *,