Tests: Simplify Django version compatibility

* Restructure runtests.py as suggested in
  https://docs.djangoproject.com/en/1.9/topics/testing/advanced/#using-the-django-test-runner-to-test-reusable-applications

* Load version-specific Django settings modules
  (rather than trying to make a single settings.configure()
  work with all supported versions).

* Use `django-admin startproject` defaults for all
  settings modules. (Tests compatibility with typical
  apps, middleware, and other settings.)

* Set up tests-specific url config; switch to literal
  urls in test cases. (Eliminates url `reverse` from
  tests.)

* Make runtests.py executable

Closes #18

runtests.py

@@ -1,57 +1,33 @@
+#!/usr/bin/env python
+
 # python setup.py test
 #   or
-# python runtests.py [anymail.tests.test_x anymail.tests.test_y.SomeTestCase ...]
+# runtests.py [tests.test_x tests.test_y.SomeTestCase ...]
 
 import sys
+
+import django
+import os
 import warnings
-
-from django import setup
 from django.conf import settings
-from django.test.runner import DiscoverRunner as TestRunner
-
-warnings.simplefilter('default')  # show DeprecationWarning and other default-ignored warnings
-
-APP = 'anymail'
-
-settings.configure(
-    DEBUG=True,
-    DATABASES={
-        'default': {
-            'ENGINE': 'django.db.backends.sqlite3',
-        }
-    },
-    ROOT_URLCONF=APP+'.urls',
-    INSTALLED_APPS=(
-        'django.contrib.auth',
-        'django.contrib.contenttypes',
-        'django.contrib.sessions',
-        'django.contrib.admin',
-        APP,
-    ),
-    MIDDLEWARE_CLASSES=(
-        'django.middleware.common.CommonMiddleware',
-        'django.contrib.sessions.middleware.SessionMiddleware',
-        'django.middleware.csrf.CsrfViewMiddleware',
-        'django.contrib.auth.middleware.AuthenticationMiddleware',
-    ),
-    TEMPLATES=[
-        # Anymail doesn't have any templates, but tests need a TEMPLATES
-        # setting to avoid warnings from the Django 1.8+ test client.
-        {
-            'BACKEND': 'django.template.backends.django.DjangoTemplates',
-        },
-    ],
-)
-
-# Initialize Django app registry
-setup()
+from django.test.utils import get_runner
 
 
-def runtests(*args):
+def runtests(test_labels=None):
+    """Discover and run project tests. Returns number of failures."""
+    test_labels = test_labels or ['tests']
+
+    # noinspection PyStringFormat
+    os.environ['DJANGO_SETTINGS_MODULE'] = \
+        'tests.test_settings.settings_%d_%d' % django.VERSION[:2]
+    django.setup()
+
+    TestRunner = get_runner(settings)
     test_runner = TestRunner(verbosity=1)
-    test_labels = args if len(args) > 0 else ['tests']
-    failures = test_runner.run_tests(test_labels)
-    sys.exit(failures)
+    return test_runner.run_tests(test_labels)
+
 
 if __name__ == '__main__':
-    runtests(*sys.argv[1:])
+    warnings.simplefilter('default')  # show DeprecationWarning and other default-ignored warnings
+    failures = runtests(test_labels=sys.argv[1:])
+    sys.exit(bool(failures))