use expected tests/settings.py file (and database) (#13079)

tests/settings.py

@@ -3,7 +3,7 @@ import os
 LANGUAGE_CODE = 'en'
 DATABASES = {
     'default': {
-        'ENGINE': os.environ['DB_ENGINE'],
+        'ENGINE': 'django.db.backends.%s' % os.environ['DB_ENGINE'],
         'TEST': {
             'NAME': 'a2-test',
         },

tox.ini

@@ -11,7 +11,7 @@ envlist = {coverage,nocoverage}-{dj17,dj18,dj19}-{pg,sqlite},package
 whitelist_externals =
     /bin/mv
 setenv =
-    AUTHENTIC2_SETTINGS_FILES=tests/settings.py
+    AUTHENTIC2_SETTINGS_FILE=tests/settings.py
     DJANGO_SETTINGS_MODULE=authentic2.settings
     sqlite: DB_ENGINE=sqlite3
     pg: DB_ENGINE=postgresql_psycopg2