Fail fast if the database doesn't support transactions.

This also fixes Django 1.6, as it tries to run
connection.features.supports_transactions after our Atomic.__enter__,
and it fails.

Author: ktosiek

pytest_django/fixtures.py

@@ -205,7 +205,8 @@ def shared_db_wrapper(_django_db_setup, _django_cursor_wrapper):
     This fixture provides a context manager that let's you access the database
     from a transaction spanning multiple tests.
     """
-    from django.db import transaction
+    from django.db import connection, transaction
+
     if get_django_version() < (1, 6):
         raise Exception('shared_db_wrapper is only supported on Django >= 1.6.')
 
@@ -220,6 +221,10 @@ def wrapper(request):
             raise Exception(
                 'shared_db_wrapper cannot be used with `transactional_db`.')
 
+        with _django_cursor_wrapper:
+            if not connection.features.supports_transactions:
+                raise Exception("Database doesn't support transactions.")
+
         exc_type, exc_value, traceback = DummyException, DummyException(), None
         # Use atomic instead of calling .savepoint* directly.
         # This way works for both top-level transactions and "subtransactions".