Add spec that update epics works with subgroups

Description

Looks like we introduced a bug when migrating from Rest calls to GraphQL calls. Let's update our feature specs to include updating epic within subgroup (beefing our unit tests in actions_spec would be nice too) 😄

Context

The following discussion from !23289 (merged) should be addressed:

• @pslaughter started a discussion:

  test (non-blocking): I confirmed locally that this fix works, but it'd be nice to have a spec for this kind of thing... Because of the time sensitivity of this, I'll create a ~"technical debt" follow up for this spec.