Fixed #10250 -- Added a regression test to ensure that GROUP BY statements are correctly quoted under MySQL. This appears to have been corrected inadvertently since the original report, but the extra regression test will make sure it stays that way.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10043 bcc190cf-cafb-0310-a4f2-bffc1f526a37