Fixed #19192 -- Allowed running tests with dummy db backend

Thanks Simon Charette for the initial patch, and Jan Bednařík for
his work on the ticket.

AUTHORS

@@ -126,6 +126,7 @@ answer newbie questions, and generally made Django that much better:
     Chris Chamberlin <dja@cdc.msbx.net>
     Amit Chakradeo <http://amit.chakradeo.net/>
     ChaosKCW
+    Simon Charette <charette.s@gmail.com>
     Kowito Charoenratchatabhan <kowito@felspar.com>
     Sengtha Chay <sengtha@e-khmer.com>
     ivan.chelubeev@gmail.com

django/db/backends/dummy/base.py

@@ -31,6 +31,10 @@ class DatabaseOperations(BaseDatabaseOperations):
 class DatabaseClient(BaseDatabaseClient):
     runshell = complain
 
+class DatabaseCreation(BaseDatabaseCreation):
+    create_test_db = ignore
+    destroy_test_db = ignore
+
 class DatabaseIntrospection(BaseDatabaseIntrospection):
     get_table_list = complain
     get_table_description = complain
@@ -64,6 +68,6 @@ def __init__(self, *args, **kwargs):
         self.features = BaseDatabaseFeatures(self)
         self.ops = DatabaseOperations(self)
         self.client = DatabaseClient(self)
-        self.creation = BaseDatabaseCreation(self)
+        self.creation = DatabaseCreation(self)
         self.introspection = DatabaseIntrospection(self)
         self.validation = BaseDatabaseValidation(self)

tests/regressiontests/test_runner/tests.py

@@ -261,6 +261,24 @@ def test_transaction_support(self):
                 db.connections = old_db_connections
 
 
+class DummyBackendTest(unittest.TestCase):
+    def test_setup_databases(self):
+        """
+        Test that setup_databases() doesn't fail with dummy database backend.
+        """
+        runner = DjangoTestSuiteRunner(verbosity=0)
+        old_db_connections = db.connections
+        try:
+            db.connections = db.ConnectionHandler({})
+            old_config = runner.setup_databases()
+            runner.teardown_databases(old_config)
+        except Exception as e:
+            self.fail("setup_databases/teardown_databases unexpectedly raised "
+                      "an error: %s" % e)
+        finally:
+            db.connections = old_db_connections
+
+
 class AutoIncrementResetTest(TransactionTestCase):
     """
     Here we test creating the same model two times in different test methods,