Simplify model validation specs for `CustomFilter`

spec/models/custom_filter_spec.rb

@@ -7,19 +7,7 @@ RSpec.describe CustomFilter do
     it { is_expected.to validate_presence_of(:title) }
     it { is_expected.to validate_presence_of(:context) }
 
-    it 'requires non-empty of context' do
-      record = described_class.new(context: [])
-      record.valid?
-
-      expect(record).to model_have_error_on_field(:context)
-    end
-
-    it 'requires valid context value' do
-      record = described_class.new(context: ['invalid'])
-      record.valid?
-
-      expect(record).to model_have_error_on_field(:context)
-    end
+    it { is_expected.to_not allow_values([], %w(invalid)).for(:context) }
   end
 
   describe 'Normalizations' do