Run the test suite on GitHub Actions

.github/workflows/tests.yml

@@ -0,0 +1,38 @@
+name: Tests
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+
+jobs:
+  rspec:
+    name: RSpec (Ruby ${{ matrix.ruby }})
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: ['3.0', '3.1', '3.2', '3.3', '3.4']
+        os: [ubuntu-latest]
+        include:
+          # Ruby 2.7 is not available on the ubuntu-24.04 runner image.
+          - ruby: '2.7'
+            os: ubuntu-22.04
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+
+      # Procodile has no runtime dependencies outside the standard library, and the
+      # committed Gemfile.lock pins rspec 3.5.x (2016), which will not install on
+      # current Rubies. Installing rspec directly keeps every Ruby in the matrix
+      # working without needing to touch the lockfile.
+      - name: Install rspec
+        run: gem install rspec --no-document
+
+      - name: Run tests
+        run: rspec -Ilib