Fixed error introduced in r14666 that results in the message reporting the...

Fixed error introduced in r14666 that results in the message reporting the test DB name to be created being shown actually after the real action when the operation fails.
Implemented that by factoring out calculation of test DB name to an internal method.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14861 bcc190cf-cafb-0310-a4f2-bffc1f526a37