spec: group_spec for rails3

the Group#infer_defaults method resets the account based on the
context. the old test only passed because of caching. assigning
the context will cause the account to be assigned on save.

Change-Id: Ie59c97605802ce208649106c89f75aada62bd702
Reviewed-on: https://gerrit.instructure.com/29527
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>
QA-Review: Jon Willesen <jonw@instructure.com>