Try an older Minitest version

Author: Mr0grog

.github/workflows/ci.yml

@@ -23,6 +23,8 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
+      - name: Install compatible Minitest
+        run: bundle add minitest --version 5.9.0
       - name: Run tests
         run: ruby -Ilib:test test/*