add migration spec

spec/migrations/remove_project_labels_group_id_spec.rb

+# encoding: utf-8
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20180202111106_remove_project_labels_group_id.rb')
+
+describe RemoveProjectLabelsGroupId, :delete do
+  let(:migration) { described_class.new }
+  let(:group) { create(:group) }
+  let!(:project_label) { create(:label, group_id: group.id) }
+  let!(:group_label) { create(:group_label) }
+
+  describe '#up' do
+    it 'updates the project labels group ID' do
+      expect { migration.up }.to change { project_label.reload.group_id }.from(1).to(nil)
+    end
+
+    it 'keeps the group labels group ID' do
+      expect { migration.up }.not_to change { group_label.reload.group_id }
+    end
+  end
+end