Add support for DATABASE_SUPPORTS_TRANSACTIONS to django.contrib.gis.tests.run_tests

[Malcolm Tredinnick]

After r9756, it's no longer possible to run a full test suite for a set of applications (a Django "project") when using the gis test runner. This is because we need to set up DATABASE_SUPPORTS_TRANSACTIONS, which is used by, at a minimum, the auth tests.

This seems like the tip of the iceberg in some ways, though. There's already a handy function to help with this, but it's part of django.db.backends.creation.BaseDatabaseCreation, which django.contrib.gis doesn't use at all for creating test databases. I suspect the solution is to break up the BaseDatabaseCreation class into basic stuff and test stuff so that gis can use the subclass the latter.