Parallelise test

Runs the test suite split across nodes.
We introduce a script `test_splitter` to handle splitting up the
test suite based on the number of nodes.
I don't know of a good way to avoid having to perform the set up on each
node but as the bulk of the run time is spent on the tests this is a
worthwhile trade off IMO.

.github/workflows/ci.yaml

@@ -3,6 +3,11 @@ name: CI
 on: push
 jobs:
   test_and_lint:
+    strategy:
+      fail-fast: false
+      matrix:
+        ci_node_total: [8]
+        ci_node_index: [0, 1, 2, 3, 4, 5, 6, 7]
     runs-on: ubuntu-latest
     services:
       postgres:
@@ -18,6 +23,8 @@ jobs:
       DFE_TEACHERS_PAYMENT_SERVICE_DATABASE_USERNAME: postgres
       DFE_TEACHERS_PAYMENT_SERVICE_DATABASE_PASSWORD: password
       DFE_TEACHERS_PAYMENT_SERVICE_DATABASE_HOST: localhost
+      CI_NODE_TOTAL: ${{ matrix.ci_node_total }}
+      CI_NODE_INDEX: ${{ matrix.ci_node_index }}
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -26,7 +33,7 @@ jobs:
       - name: Setup yarn
         run: npm install -g yarn
       - name: Install Node packages
-        run: yarn install # TODO RL: cache yarn
+        run: yarn install
       - name: Install Ruby
         uses: ruby/setup-ruby@v1
         with:
@@ -42,5 +49,11 @@ jobs:
         run: bin/rails standard
       - name: Prepare DB
         run: bin/rails db:schema:load
-      - name: Run test
-        run: bundle exec rspec
+      - name: Run unit tests
+        env:
+          SPEC_TYPE: unit
+        run: bundle exec rspec $(bin/test_splitter)
+      - name: Run feature tests
+        env:
+          SPEC_TYPE: feature
+        run: bundle exec rspec $(bin/test_splitter)

bin/test_splitter

@@ -0,0 +1,26 @@
+#!/usr/bin/env ruby
+
+# Use on ci to split tests by file type and split them across ci nodes
+
+all_specs = Dir["spec/**/*_spec.rb"]
+
+case ENV["SPEC_TYPE"]
+when "unit"
+  specs = all_specs.reject { |path| /\Aspec\/features\//.match? path }
+when "feature"
+  specs = all_specs.select { |path| /\Aspec\/features\//.match? path }
+else
+  specs = all_specs
+end
+
+number_of_ci_nodes = ENV.fetch("CI_NODE_TOTAL").to_i
+
+this_ci_node = ENV.fetch("CI_NODE_INDEX").to_i
+
+number_of_slices = (specs.size / number_of_ci_nodes.to_f).ceil
+
+specs_split_across_nodes = specs.each_slice(number_of_slices).to_a
+
+specs_to_run_on_this_node = specs_split_across_nodes[this_ci_node]
+
+print specs_to_run_on_this_node.join(" ")