Add analytics dimension for A/B tests

Link meta tags named `govuk:ab-test` to a custom Google Analytics
dimension. The value of the meta tag and the dimension is a combination
of the test name and bucket name, such as `EducationNavigation:B`.

This links to existing Session dimension 13, which has been used for A/B
tests in the past.

Note that only one A/B test can use this dimension at once, otherwise
the value will be overridden by different tests.

app/assets/javascripts/analytics/static-analytics.js

@@ -94,6 +94,7 @@
     this.setOrganisationsDimension(dimensions['analytics:organisations']);
     this.setWorldLocationsDimension(dimensions['analytics:world-locations']);
     this.setRenderingApplicationDimension(dimensions['rendering-application']);
+    this.setAbTestDimension(dimensions['ab-test']);
   };
 
   StaticAnalytics.prototype.trackPageview = function(path, title, options) {
@@ -156,5 +157,9 @@
     this.setDimension(21, position);
   };
 
+  StaticAnalytics.prototype.setAbTestDimension = function(testNameAndBucket) {
+    this.setDimension(40, testNameAndBucket);
+  };
+
   GOVUK.StaticAnalytics = StaticAnalytics;
 })();

spec/javascripts/analytics/static-analytics-spec.js

@@ -62,6 +62,7 @@ describe("GOVUK.StaticAnalytics", function() {
           <meta name="govuk:political-status" content="historic">\
           <meta name="govuk:analytics:organisations" content="<D10>">\
           <meta name="govuk:analytics:world-locations" content="<W1>">\
+          <meta name="govuk:ab-test" content="name-of-test:name-of-ab-bucket">\
         ');
 
         analytics = new GOVUK.StaticAnalytics({universalId: 'universal-id'});
@@ -74,6 +75,7 @@ describe("GOVUK.StaticAnalytics", function() {
         expect(universalSetupArguments[8]).toEqual(['set', 'dimension7', 'historic']);
         expect(universalSetupArguments[9]).toEqual(['set', 'dimension9', '<D10>']);
         expect(universalSetupArguments[10]).toEqual(['set', 'dimension10', '<W1>']);
+        expect(universalSetupArguments[11]).toEqual(['set', 'dimension40', 'name-of-test:name-of-ab-bucket']);
       });
 
       it('ignores meta tags not set', function() {