define PHPUNIT constant only if not already set

tests/bootstrap.php

@@ -3,7 +3,9 @@
 global $RUNTIME_NOAPPS;
 $RUNTIME_NOAPPS = true;
 
-define('PHPUNIT_RUN', 1);
+if (!defined('PHPUNIT_RUN')) {
+	define('PHPUNIT_RUN', 1);
+}
 
 require_once __DIR__.'/../../../lib/base.php';