tests: added test that Environment::RUNNER is present

Author: milo

tests/Runner/Runner.environment.phpt

@@ -0,0 +1,8 @@
+<?php
+
+use Tester\Assert;
+use Tester\Environment;
+
+require __DIR__ . '/../bootstrap.php';
+
+Assert::same('1', getenv(Environment::RUNNER));