build: add PHPUnit dependency

phpunit.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit backupGlobals="false"
+         backupStaticAttributes="false"
+         bootstrap="./vendor/autoload.php"
+         colors="true"
+         convertErrorsToExceptions="true"
+         convertNoticesToExceptions="true"
+         convertWarningsToExceptions="true"
+         processIsolation="false"
+         stopOnFailure="false"
+>
+    <testsuites>
+        <testsuite name="Application Test Suite">
+            <directory>./tests/</directory>
+        </testsuite>
+    </testsuites>
+</phpunit>