Only state variant if content navigation test applies

The helper methods for checking the current variant (eg
`universal_navigation_without_nav?`) were not checking whether the AB
test applied or not.

If the CDN set the response header and this method was used on a page
that doesn’t qualify for the test, then it would give a false positive.

* Add test to confirm expected behaviour
* Update helpers to check AB test applies before checking variant

app/controllers/concerns/content_navigation_ab_testable.rb

@@ -1,4 +1,5 @@
 module ContentNavigationABTestable
+  CONTENT_NAVIGATION_TEST_NAME = "ContentNavigation".freeze
   CONTENT_NAVIGATION_DIMENSION = 67
   CONTENT_NAVIGATION_ORIGINAL = "Original".freeze
   CONTENT_NAVIGATION_UNIVERSAL_NO_NAVIGATION = "UniversalNoNav".freeze
@@ -23,7 +24,6 @@ def self.included(base)
     base.helper_method(
       :content_navigation_variant,
       :content_navigation_ab_test_applies?,
-      :content_navigation_test_original_variant?,
       :universal_navigation_without_nav?,
       :universal_navigation_with_taxon_nav?,
       :universal_navigation_with_mainstream_nav?
@@ -35,7 +35,7 @@ def self.included(base)
   def content_navigation_ab_test
     @content_navigation_ab_test ||=
       GovukAbTesting::AbTest.new(
-        "ContentNavigation",
+        CONTENT_NAVIGATION_TEST_NAME,
         dimension: CONTENT_NAVIGATION_DIMENSION,
         allowed_variants: CONTENT_NAVIGATION_ALLOWED_VARIANTS,
         control_variant: CONTENT_NAVIGATION_ORIGINAL
@@ -49,7 +49,7 @@ def content_navigation_ab_test
   # * in the multivariate test
   # * not showing the original variant
   def should_present_universal_navigation?
-    content_navigation_ab_test_applies? && !content_navigation_test_original_variant?
+    content_navigation_ab_test_applies? && !content_navigation_variant.variant?(CONTENT_NAVIGATION_ORIGINAL)
   end
 
   def content_navigation_ab_test_applies?
@@ -60,20 +60,19 @@ def content_document_type_is_guidance?
     GUIDANCE_DOCUMENT_TYPES.include? @content_item.document_type
   end
 
-  def content_navigation_test_original_variant?
-    content_navigation_variant.variant?(CONTENT_NAVIGATION_ORIGINAL)
-  end
-
   def universal_navigation_without_nav?
-    content_navigation_variant.variant?(CONTENT_NAVIGATION_UNIVERSAL_NO_NAVIGATION)
+    content_navigation_ab_test_applies? &&
+      content_navigation_variant.variant?(CONTENT_NAVIGATION_UNIVERSAL_NO_NAVIGATION)
   end
 
   def universal_navigation_with_taxon_nav?
-    content_navigation_variant.variant?(CONTENT_NAVIGATION_UNIVERSAL_TAXON_NAVIGATION)
+    content_navigation_ab_test_applies? &&
+      content_navigation_variant.variant?(CONTENT_NAVIGATION_UNIVERSAL_TAXON_NAVIGATION)
   end
 
   def universal_navigation_with_mainstream_nav?
-    content_navigation_variant.variant?(CONTENT_NAVIGATION_UNIVERSAL_MAINSTREAM_NAVIGATION)
+    content_navigation_ab_test_applies? &&
+      content_navigation_variant.variant?(CONTENT_NAVIGATION_UNIVERSAL_MAINSTREAM_NAVIGATION)
   end
 
   def content_navigation_variant

test/controllers/content_navigation_ab_testing_content_items_controller_test.rb

@@ -67,5 +67,25 @@ class ContentItemsControllerTest < ActionController::TestCase
       assert_equal [], @request.variant
       assert_equal ContentItemsController::CONTENT_NAVIGATION_ORIGINAL, requested_variant_name
     end
+
+    test "#{document_type} does not mark itself as in variant when ab test does not apply" do
+      content_item = content_store_has_schema_example(schema_name, schema_name)
+      path = "government/abtest/#{schema_name}"
+      content_item['document_type'] = document_type
+      content_item['base_path'] = "/#{path}"
+      content_item['links'] = {}
+      content_store_has_item(content_item['base_path'], content_item)
+
+      ContentItemsController::CONTENT_NAVIGATION_ALLOWED_VARIANTS.each do |variant|
+        request.headers["GOVUK-ABTest-#{ContentItemsController::CONTENT_NAVIGATION_TEST_NAME}"] = variant
+        get :show, params: { path: path }
+        assert_response 200
+        refute @controller.content_navigation_ab_test_applies?
+
+        refute @controller.universal_navigation_without_nav?
+        refute @controller.universal_navigation_with_taxon_nav?
+        refute @controller.universal_navigation_with_mainstream_nav?
+      end
+    end
   end
 end