Parallelise MiniTest workflow

This configures the MiniTest workflow to run
using a matrix strategy. It run separate parts
of the test suite on different nodes and hence
speeds up total run time of the CI/CD pipeline.

.github/workflows/minitest.yml

@@ -17,6 +17,14 @@ jobs:
   run-minitest:
     name: Run Minitest
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ci_node_index: [
+          "test/unit/lib test/unit/app/helpers test/unit/app/presenters test/unit/app/validators test/unit/app/workers",
+          "test/unit/app/models test/unit/app/services test/unit/app/uploaders",
+          "test/functional test/components test/integration test/views"
+        ]
     steps:
       - name: Setup MySQL
         id: setup-mysql
@@ -65,4 +73,4 @@ jobs:
           RAILS_ENV: test
           GOVUK_CONTENT_SCHEMAS_PATH: vendor/publishing-api/content_schemas
           TEST_DATABASE_URL: ${{ steps.setup-mysql.outputs.db-url }}
-        run: bundle exec rake test
+        run: bundle exec rails test ${{ matrix.ci_node_index }}