Add one more test

Author: e1himself

tests/it_should_implement_psr_container_interface.php

@@ -0,0 +1,8 @@
+<?php
+
+use Psr\Container\ContainerInterface;
+use Technically\NullContainer\NullContainer;
+
+$container = new NullContainer();
+
+assert($container instanceof ContainerInterface);