Update AA test config for ElasticSearch 6.7

There is currently an AA test running with a 50/50 split [1].
We now want to run a throttled version with a 0.5% split.
The C variant has therefore been added to the corresponding
govuk-cdn-config PR to soak up the traffic not in A or B.
This PR allows the C variant. Z will remain as the control variant.
[1]#3135

app/controllers/ab_tests/elastic_search_aa_testable.rb

@@ -3,7 +3,7 @@ def elastic_search_aa_test
     GovukAbTesting::AbTest.new(
       "EsSixPointSeven",
       dimension: 41,
-      allowed_variants: %w[A B Z],
+      allowed_variants: %w[A B C Z],
       control_variant: "Z",
     )
   end

spec/controllers/finders_controller_spec.rb

@@ -111,7 +111,7 @@
       end
 
       context "with AA test" do
-        %w[A B Z].each do |variant|
+        %w[A B C Z].each do |variant|
           it "renders the #{variant} variant for /search/all pages" do
             stub_content_store_has_item(
               "/search/all",