[1.8.x] Fixed #25079 -- Added warning if both TEMPLATES and TEMPLATE_* settings are defined.

Django ignores the value of the TEMPLATE_* settings if TEMPLATES is also
set, which is confusing for users following older tutorials. This change
adds a system check that warns if any of the TEMPLATE_* settings have
changed from their defaults but the TEMPLATES dict is also non-empty.

Removed the TEMPLATE_DIRS from the test settings file; this was marked
for removal in 1.10 but no tests fail if it is removed now.

Backport of 24620d71f2 from master

tests/check_framework/tests_1_8_compatibility.py

@@ -0,0 +1,29 @@
+from django.core.checks.compatibility.django_1_8_0 import \
+    check_duplicate_template_settings
+from django.test import SimpleTestCase
+from django.test.utils import override_settings
+
+
+class CheckDuplicateTemplateSettingsTest(SimpleTestCase):
+
+    def test_not_raised_if_no_templates_setting(self):
+        self.assertEqual(check_duplicate_template_settings(None), [])
+
+    @override_settings(
+        TEMPLATES=[{'BACKEND': 'django.template.backends.django.DjangoTemplates'}],
+        TEMPLATE_DIRS=['/path/to/dirs'],
+    )
+    def test_duplicate_setting(self):
+        result = check_duplicate_template_settings(None)
+        self.assertEqual(result[0].id, '1_8.W001')
+
+    @override_settings(
+        TEMPLATES=[{'BACKEND': 'django.template.backends.django.DjangoTemplates'}],
+        TEMPLATE_DIRS=['/path/to/dirs'],
+        TEMPLATE_DEBUG=True,
+    )
+    def test_multiple_duplicate_settings(self):
+        result = check_duplicate_template_settings(None)
+        self.assertEqual(len(result), 1)
+        self.assertTrue('TEMPLATE_DIRS' in result[0].msg)
+        self.assertTrue('TEMPLATE_DEBUG' in result[0].msg)