tests(github): Enable Github Actions testing (#131)

.github/workflows/testing.yml

+name: Tests
+on:
+  push:
+    branches: [ 8.3.x, 8.x-3.x ]
+  pull_request:
+    branches: [ 8.3.x, 8.x-3.x ]
+jobs:
+  testing:
+    name: PHP ${{ matrix.php-versions }}
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        php-versions: ['7.1', '7.2', '7.3', '7.4']
+        phpstan: ['1']
+        # Extra run to also test on PHP 7.0 without PHPStan.
+        include:
+          - php-versions: '7.0'
+            phpstan: '0'
+    steps:
+      - name: Checkout Coder
+        uses: actions/checkout@v2
+      - name: Setup PHP, with composer and extensions
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-versions }}
+          extensions: mbstring
+      - name: Get composer cache directory
+        id: composercache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+      - name: Cache composer dependencies
+        uses: actions/cache@v2
+        with:
+          path: ${{ steps.composercache.outputs.dir }}
+          # Use composer.json for key, if composer.lock is not committed.
+          key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.json') }}
+          restore-keys: ${{ runner.os }}-composer-
+      - name: Install Composer dependencies
+        # Running composer install without a lock file will also update cached
+        # dependencies in vendor.
+        # We have a dedicated composer.json file for PHP 7.0 that does not have
+        # PHPStan because it requires >= PHP 7.1.
+        run: |
+          if [[ ${{ matrix.php-versions }} == "7.0" ]]; then cp composer-php-7-0.json composer.json; fi
+          composer install --no-progress --prefer-dist --optimize-autoloader
+      - name: Run PHPUnit
+        run: ./vendor/bin/phpunit
+      - name: Run PHPCS
+        run: ./vendor/bin/phpcs
+      - name: Check custom standard autoloading
+        # Ensure that a custom standard can be invoked and the auto-loading of
+        # abstract classes works.
+        # Ensure that the DrupalPractice standard can be invoked standalone and the
+        # auto-loading of abstract classes works.
+        run: |
+          ./vendor/bin/phpcs -p --standard=tests/Drupal/phpcs-ruleset.xml tests/Drupal/good/ --ignore=tests/Drupal/good/GoodUnitTest.php
+          ./vendor/bin/phpcs -p --standard=coder_sniffer/DrupalPractice tests/DrupalPractice/good/ --ignore=tests/DrupalPractice/good/GoodUnitTest.php
+      - name: Run PHPStan
+        run: if [[ ${{ matrix.phpstan }} == "1" ]]; then ./vendor/bin/phpstan analyse; fi