Parallelize Ruby tests in CI build

This is closely based on alphagov/whitehall#8364.

I've put `minitest-ci` in the `scripts` directory rather than the `bin`
directory, because I think the latter is usually reserved for built-in
scripts.

I've made `minitest-ci` executable in the git repo rather than having a
separate step in the workflow job.

I've run rubocop auto-correct against `minitest-ci` to fix some
violations.

I haven't bothered to split the Ruby tests out into a separate workflow
file for now.

.github/workflows/ci.yml

@@ -37,9 +37,13 @@ jobs:
     with:
       useWithRails: true
 
-  test-ruby:
-    name: Test Ruby
+  test-ruby-matrix:
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ci_node_total: [4]
+        ci_node_index: [0, 1, 2, 3]
     steps:
       - name: Setup MySQL
         id: setup-mysql
@@ -72,5 +76,13 @@ jobs:
         env:
           RAILS_ENV: test
           TEST_DATABASE_URL: ${{ steps.setup-mysql.outputs.db-url }}
-        run: bundle exec rake test
+          CI_NODE_TOTAL: ${{ matrix.ci_node_total }}
+          CI_NODE_INDEX: ${{ matrix.ci_node_index }}
+        run: ./script/minitest-ci
 
+  test-ruby:
+    name: Test Ruby
+    needs: test-ruby-matrix
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo "All Minitest tests have passed 🚀"

script/minitest-ci

@@ -0,0 +1,16 @@
+#!/usr/bin/env ruby
+
+# This script is used during the GitHub Actions workflow
+# defined in .github/workflows/ci.yml.
+# It splits the MiniTest suite into randomly-allocated groups
+# which are executed across multiple GitHub Actions 'matrix' nodes.
+
+tests = Dir["test/**/*_test.rb"]
+  .sort
+  .shuffle(random: Random.new(ENV["GITHUB_SHA"].to_i(16)))
+  .select
+  .with_index do |_el, i|
+    i % ENV["CI_NODE_TOTAL"].to_i == ENV["CI_NODE_INDEX"].to_i
+  end
+
+exec "bundle exec rails test #{tests.join(' ')}"