Fixed #32446 -- Deprecated SERIALIZE test database setting.

Whether or not the state of a test database should be serialized can be
inferred from the set of databases allowed to be access from discovered
TestCase/TransactionTestCase enabling the serialized_rollback feature
which makes this setting unnecessary.

This should make a significant test suite bootstraping time difference
on large projects that didn't explicitly disable test database
serialization.

tests/test_runner_apps/databases/tests.py

@@ -10,6 +10,11 @@ class DefaultDatabaseTests(NoDatabaseTests):
     databases = {'default'}
 
 
+class DefaultDatabaseSerializedTests(NoDatabaseTests):
+    databases = {'default'}
+    serialized_rollback = True
+
+
 class OtherDatabaseTests(NoDatabaseTests):
     databases = {'other'}