Add CI workflow

.github/workflows/ci.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - "v2.99.3-with-ruby3-compat"
+  pull_request:
+    branches:
+      - "v2.99.3-with-ruby3-compat"
+
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby-version: ['2.7']
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+
+      - name: Setup Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby-version }}
+          bundler-cache: true
+
+      - name: Run tests
+        run: |
+          bundle exec rake test