Add spec for GraphQL note creation for Epics

https://gitlab.com/gitlab-org/gitlab-ce/issues/62826

ee/spec/requests/api/graphql/mutations/notes/create/note_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Adding a Note to an Epic' do
+  include GraphqlHelpers
+
+  set(:current_user) { create(:user) }
+  let(:epic) { create(:epic, group: group) }
+  let(:mutation) do
+    variables = {
+      noteable_id: GitlabSchema.id_from_object(epic).to_s,
+      body: 'Body text'
+    }
+
+    graphql_mutation(:create_note, variables)
+  end
+
+  def mutation_response
+    graphql_mutation_response(:create_note)
+  end
+
+  before do
+    stub_licensed_features(epics: true)
+  end
+
+  context 'when the user does not have permission' do
+    let(:group) { create(:group, :private) }
+
+    it_behaves_like 'a Note mutation when the user does not have permission'
+  end
+
+  context 'when the user has permission' do
+    let(:group) { create(:group, :public) }
+
+    it_behaves_like 'a Note mutation that creates a Note'
+  end
+end