More robust replacement of AB test content

As suggested by @hannako, we can make the AB test replacement less
fragile by looking for a specific replacement placeholder in the content
item, instead of replacing a particular bit of content.

I've gone for a [mustache](https://mustache.github.io/) style
replacement string of {{ab_test_find_utr_number_video_links}}. There's a
possible future thought of doing other kinds of templating, so I think
starting to socialise this syntax is a good idea.

I've checked in the govspeak preview app, and the `{{...}}` syntax is ignored
by govspeak so there should be no issue entering it in the publishing
app.

I've pulled the replacement strings out into the translation file so
that they're easier to review / keep up to date.

I've also switched to three A / B / Z variants, so we can do splits like
5% / 5% / 90% where we're measuring the difference between the two 5%
samples and ignoring the 90% sample. This test is probably going to be
50% / 50%, so not strictly necessary, but this seems to be the idiomatic
way to do it.

app/controllers/content_items_controller.rb

@@ -30,26 +30,21 @@ def show
       ab_test = GovukAbTesting::AbTest.new(
         "find_utr_number_video_links",
         dimension: 300, # TODO: which dimension should we use?
-        allowed_variants: %w[HelpText VideoLink],
-        control_variant: "HelpText",
+        allowed_variants: %w[A B Z],
+        control_variant: "Z",
       )
       @requested_variant = ab_test.requested_variant(request.headers)
       @requested_variant.configure_response(response)
 
-      if @requested_variant.variant? "VideoLink"
-        # NOTE: this is a fragile way of doing an AB test on content.
-        #
-        # Any change to the base content, or to the way the content is rendered
-        # could potentially break the B variant of the test, and result in both
-        # variants being the same.
-        # We're aware of this risk, and we're going to be careful in this one off
-        # situation. This is not a sustainable way of doing AB tests in the
-        # future.
-        @content_item.body.sub!(
-          /<li>\s*in\ the\s+<a\ href="[^"]*"><abbr\ title="[^"]+">HMRC<\/abbr>\s+app<\/a>\s*<\/li>/,
-          '<li>in the <a href="https://www.gov.uk/guidance/download-the-hmrc-app"><abbr title="HM Revenue and Customs">HMRC</abbr> app</a> - watch a <a href="https://www.youtube.com/watch?v=LXw9ily9rTo">video about finding your UTR number in the app</a></li>',
-        )
-      end
+      replacement = case @requested_variant.variant_name
+                    when "A"
+                      I18n.t("ab_tests.find_utr_number_video_links.A")
+                    when "B"
+                      I18n.t("ab_tests.find_utr_number_video_links.B")
+                    else
+                      I18n.t("ab_tests.find_utr_number_video_links.Z")
+                    end
+      @content_item.body.sub!("{{ab_test_find_utr_number_video_links}}", replacement)
     end
     # /TEMPORARY
 

config/locales/en.yml

@@ -1,5 +1,10 @@
 ---
 en:
+  ab_tests:
+    find_utr_number_video_links:
+      A: 'in the <a href="/guidance/download-the-hmrc-app"><abbr title="HM Revenue and Customs">HMRC</abbr> app</a>'
+      B: 'in the <abbr title="HM Revenue and Customs">HMRC</abbr> app - watch a <a href="https://www.youtube.com/watch?v=LXw9ily9rTo">video about finding your UTR number in the app</a>'
+      Z: 'in the <a href="/guidance/download-the-hmrc-app"><abbr title="HM Revenue and Customs">HMRC</abbr> app</a>'
   call_for_evidence:
     and: and
     another_website_html: This call for evidence %{closed} held on <a href="%{url}">another website</a>

test/controllers/content_items_controller_test.rb

@@ -364,19 +364,36 @@ class ContentItemsControllerTest < ActionController::TestCase
     assert_equal "true", @response.headers[Slimmer::Headers::REMOVE_SEARCH_HEADER]
   end
 
-  test "AB test replaces content on the find-utr-number page" do
+  test "AB test replaces content on the find-utr-number page with default" do
     content_item = content_store_has_schema_example("answer", "answer")
     content_item["base_path"] = "/find-utr-number"
-    content_item["details"]["body"] = "<li>in the <a href=\"\"><abbr title=\"HM Revenue and Customs\">HMRC</abbr> app</a>\n</li>"
+    content_item["details"]["body"] = "<li>{{ab_test_find_utr_number_video_links}}</li>"
     content_item["locale"] = "en"
 
     stub_content_store_has_item(content_item["base_path"], content_item)
 
-    request.headers["HTTP_GOVUK_ABTEST_FIND_UTR_NUMBER_VIDEO_LINKS"] = "VideoLink"
+    request.headers["HTTP_GOVUK_ABTEST_FIND_UTR_NUMBER_VIDEO_LINKS"] = nil
 
     get :show, params: { path: path_for(content_item) }
     assert_response :success
-    assert_match 'watch a <a href="https://www.youtube.com/watch?v=LXw9ily9rTo">video about finding your UTR number in the app</a>', response.body
+    assert_no_match "{{ab_test_find_utr_number_video_links}}", response.body
+    assert_match "<li>#{I18n.t('ab_tests.find_utr_number_video_links.Z')}</li>", response.body
+  end
+
+  test "AB test replaces content on the find-utr-number page with variant B" do
+    content_item = content_store_has_schema_example("answer", "answer")
+    content_item["base_path"] = "/find-utr-number"
+    content_item["details"]["body"] = "<li>{{ab_test_find_utr_number_video_links}}</li>"
+    content_item["locale"] = "en"
+
+    stub_content_store_has_item(content_item["base_path"], content_item)
+
+    request.headers["HTTP_GOVUK_ABTEST_FIND_UTR_NUMBER_VIDEO_LINKS"] = "B"
+
+    get :show, params: { path: path_for(content_item) }
+    assert_response :success
+    assert_no_match "{{ab_test_find_utr_number_video_links}}", response.body
+    assert_match "<li>#{I18n.t('ab_tests.find_utr_number_video_links.B')}</li>", response.body
   end
 
   def path_for(content_item, locale = nil)