Add coverage for misc serializers (#32781)

spec/serializers/rest/custom_emoji_serializer_spec.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::CustomEmojiSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  let(:record) { Fabricate.build :custom_emoji, id: 123, category: Fabricate(:custom_emoji_category, name: 'Category Name') }
+
+  describe 'serialization' do
+    it 'returns expected values' do
+      expect(subject)
+        .to include(
+          'category' => be_a(String).and(eq('Category Name'))
+        )
+    end
+  end
+end