Refactor billing plan specs

The following discussion from !63731 (merged) should be addressed:

    > ee/spec/features/billings/billing_plans_spec.rb:25:    stub_full_request("#{EE::SUBSCRIPTIONS_URL}/gitlab_plans?plan=#{plan.name}&namespace_id=#{namespace.id}")
    > ee/spec/features/billings/billing_plans_spec.rb:464:          stub_full_request("#{EE::SUBSCRIPTIONS_URL}/gitlab_plans?plan=free&namespace_id=#{namespace.id}")