Add tests for what happens on `PendingMembershipExpired`
It seems that function expire_pending_memberships
which triggers T::OnEvent::on_event(&sp_membership::Event::PendingMembershipExpired
lead to Self::dispatch_idty_call(pallet_identity::Call::remove_identity
no matter the instance of the membership pallet (simple membership or smith membership).
We have to add tests to cover this behavior and make sure that pending smith membership expiration does not kill the associated identity.