Fixed #15026 -- Added cleanup to the invalid key session tests; when using Memcached as a cache backend, the cache-backed session backends would fail on the second run due to leftover cache artefacts. Thanks to jsdalton for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15235 bcc190cf-cafb-0310-a4f2-bffc1f526a37