[1.2.X] Fixed #15029 -- Moved to database backends the ability to decide if...

[1.2.X] Fixed #15029 -- Moved to database backends the ability to decide if two DATABASES items are different when creating temporary databases for tests.

Backport of [15392] from trunk

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15395 bcc190cf-cafb-0310-a4f2-bffc1f526a37