[1.2.X] Fixed some of the problems with aggregation_regress tests on higher...

[1.2.X] Fixed some of the problems with aggregation_regress tests on higher levles of Postgres. Refs #14246. 

r13709 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13710 bcc190cf-cafb-0310-a4f2-bffc1f526a37