Reverted r16386 because it replaced a brittle method with another not less

arbitrary method when the test client checks for the presence of the bundled
session backends+session middleware combination.

We will revisit the issue soon, probably to make these checks even less strict.

Refs #7836, #16605

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17739 bcc190cf-cafb-0310-a4f2-bffc1f526a37