Add a test for Custom Emoji create mutation

There was no test coverage on the create mutation of the custom emoji
GraphQL endpoint.

This test verifies if the mutation is working and if the creator is set.

spec/graphql/mutations/custom_emoji/create_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Mutations::CustomEmoji::Create do
+  let_it_be(:group) { create(:group) }
+  let_it_be(:user) { create(:user) }
+  let(:args) { { group_path: group.full_path, name: 'tanuki', url: 'https://about.gitlab.com/images/press/logo/png/gitlab-icon-rgb.png' } }
+
+  before do
+    group.add_developer(user)
+  end
+
+  describe '#resolve' do
+    subject(:resolve) { described_class.new(object: nil, context: { current_user: user }, field: nil).resolve(**args) }
+
+    it 'creates the custom emoji' do
+      expect { resolve }.to change(CustomEmoji, :count).by(1)
+    end
+
+    it 'sets the creator to be the user who added the emoji' do
+      resolve
+
+      expect(CustomEmoji.last.creator).to eq(user)
+    end
+  end
+end