Add deterministic ordering to roulette assignment table

- otherwise we get mixture on mr's of ordering.

lib/gitlab/danger/roulette.rb

@@ -24,7 +24,7 @@ module Gitlab
       #
       # @return [Array<Spin>]
       def spin(project, categories, timezone_experiment: false)
-        spins = categories.map do |category|
+        spins = categories.sort.map do |category|
           including_timezone = INCLUDE_TIMEZONE_FOR_CATEGORY.fetch(category, timezone_experiment)
 
           spin_for_category(project, category, timezone_experiment: including_timezone)

spec/lib/gitlab/danger/roulette_spec.rb

@@ -165,6 +165,14 @@ RSpec.describe Gitlab::Danger::Roulette do
         end
       end
 
+      context 'when change contains many categories' do
+        let(:categories) { [:frontend, :test, :qa, :engineering_productivity, :ci_template, :backend] }
+
+        it 'has a deterministic sorting order' do
+          expect(spins.map(&:category)).to eq categories.sort
+        end
+      end
+
       context 'when change contains QA category' do
         let(:categories) { [:qa] }