Install standalone test dependency in CI

.github/workflows/candidate.yml

@@ -18,6 +18,8 @@ jobs:
       - uses: ruby/setup-ruby@003a5c4d8d6321bd302e38f6f0ec593f77f06600 # v1
         with:
           ruby-version: "3.4"
+      - name: Install standalone test dependency
+        run: gem install minitest --version 6.0.6 --no-document
       - run: |
           ruby test/repository_test.rb
           ruby test/runtime_tuple_promoter_test.rb

test/repository_test.rb

@@ -335,6 +335,13 @@ class RepositoryTest < Minitest::Test
     end
   end
 
+  def test_repository_job_installs_its_pinned_standalone_test_dependency
+    workflow = File.read(File.join(ROOT, ".github/workflows/candidate.yml"))
+    repository_job = workflow.split(/^  repository:\n/, 2).fetch(1).split(/^  prepare:\n/, 2).fetch(0)
+
+    assert_includes repository_job, "gem install minitest --version 6.0.6 --no-document"
+  end
+
   def test_candidate_workflow_verifies_and_preserves_the_lockstep_client_tuple
     workflow = File.read(File.join(ROOT, ".github/workflows/candidate.yml"))