Github Actions: PHPUnit

Added Github Actions runner for PHPUnit tests.
It runs on every push and makes sure all contributions (be it from humans, or bots) pass our existing tests.

.github/workflows/phpunit.yml

@@ -0,0 +1,20 @@
+name: PHPUnit
+
+on: [push]
+
+jobs:
+  build-test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: php-actions/composer@v5
+
+      - name: PHPUnit Tests
+        uses: php-actions/phpunit@v2
+        with:
+          php_extensions: xdebug mbstring
+          bootstrap: vendor/autoload.php
+          configuration: phpunit.xml
+          args: --coverage-text