1.6 compatibility checks don't correctly validate TEST_RUNNER

[Russell Keith-Magee]

The 1.6 compatibility checks (executed by the new check management command) don't appear to perform an accurate check.

I can see 5 scenarios that need to be accounted for:

1. A default pre-Django 1.6 project
2. A pre-Django 1.6 project with TEST_RUNNER set to DiscoverRunner
3. A pre-Django 1.6 project with TEST_RUNNER set to something custom
4. A default Django 1.6 project
5. A Django 1.5 project with TEST_RUNNER set to something custom

The management command does the following:

    from django.conf import settings
    new_default = 'django.test.runner.DiscoverRunner'
    test_runner_setting = getattr(settings, 'TEST_RUNNER', new_default)

    if test_runner_setting == new_default:
        ... output warning ...

That is, it retrieves the current setting value of TEST_RUNNER, and if it is DiscoverRunner, it raises a warning. This behavior:

• Is correct for case 1
• Is possibly incorrect for case 3 and 5, as there's no guarantee the custom test runner subclasses DiscoverRunner (and it probably won't)
• Incorrectly raises an error for case 2 and 4.

There are test cases for the current implementation, but they aren't very solid - they're operating inside a test environment that is itself force setting TEST_RUNNER.