Require PHPUnit for development

composer.json

@@ -7,5 +7,8 @@
     },
     "require": {
         "guzzlehttp/guzzle": "~6.0"
+    },
+    "require-dev": {
+        "phpunit/phpunit": "^8.4"
     }
 }

phpunit.xml

+<?xml version="1.0" encoding="utf-8"?>
+<phpunit bootstrap="vendor/autoload.php">
+  <testsuite name="default">
+    <directory suffix="Test.php">tests</directory>
+  </testsuite>
+</phpunit>