Fixed #16329 -- Fixed detection of transaction-handling capabilities when all test databases are sqlite3, in-memory.

Thanks canassa for the report and agriffis (#17762) and lrekucki (in #17758) for their contribution to the fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17702 bcc190cf-cafb-0310-a4f2-bffc1f526a37