Add PHPUnit configuration

phpunit.xml

+<?xml version="1.0" encoding="utf-8"?>
+<phpunit bootstrap="tests/bootstrap.php" strict="true">
+  <testsuite name="Unit tests">
+    <directory suffix="Test.php">tests/unit-tests</directory>
+  </testsuite>
+</phpunit>

tests/bootstrap.php

+<?php
+
+if ($autoload = stream_resolve_include_path('vendor/autoload.php')) {
+    require($autoload);
+}
+
+define('APP_TESTDIR', __DIR__);
\ No newline at end of file