Add coverage for StatusTrend and PreviewCardTrend models, add locales class method to RankedTrend (#32688)

spec/models/preview_card_trend_spec.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe PreviewCardTrend do
+  describe 'Associations' do
+    it { is_expected.to belong_to(:preview_card).required }
+  end
+
+  describe '.locales' do
+    before do
+      Fabricate :preview_card_trend, language: 'en'
+      Fabricate :preview_card_trend, language: 'en'
+      Fabricate :preview_card_trend, language: 'es'
+    end
+
+    it 'returns unique set of languages' do
+      expect(described_class.locales)
+        .to eq(['en', 'es'])
+    end
+  end
+end