Add GitHub actions workflow for PHPUnit

.github/workflows/tests.yml

@@ -0,0 +1,44 @@
+name: tests
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+permissions:
+  contents: read
+
+jobs:
+  phpunit:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php: [ '8.3', '8.4' ]
+        dependency-version: ['prefer-lowest', 'prefer-stable']
+    steps:
+      - uses: actions/checkout@v5
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          tools: composer:v2
+          coverage: none
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "dir=$(composer config cache-files-dir)" >> $GITHUB_OUTPUT
+
+      - name: Cache dependencies
+        uses: actions/cache@v4
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ matrix.dependency-version }}-${{ hashFiles('**/composer.json') }}
+          restore-keys: ${{ runner.os }}-composer-${{ matrix.dependency-version }}-
+
+      - name: Install dependencies
+        run: composer update --${{ matrix.dependency-version }} --prefer-dist --no-interaction
+
+      - name: Run test suite
+        run: composer test