GitHub Actions: test pull requests

.github/workflows/test.yml

@@ -0,0 +1,18 @@
+name: Ruby tests
+
+on:
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  test:
+    runs-on: ubuntu-20.04
+    steps:
+    - uses: actions/checkout@v2.3.4
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1.66.1
+      with:
+        bundler-cache: true # runs 'bundle install' and caches installed gems automatically
+    - name: Run tests
+      run: bundle exec rake