Test server tweak

tests/docroot/IsUp.php

@@ -1,4 +1,4 @@
 <?php return [
     'code'    => 200,
-    'content' => "I'm up!",
+    'content' => getmypid(),
 ];

tests/server.php

@@ -58,7 +58,7 @@ if (!file_exists($test)) {
 // set the response code
 http_response_code((int) $response['code']);
 // if the response has a body, set the content type and (possibly) the ETag.
-if (strlen($response['content'])) {
+if (strlen((string) $response['content'])) {
     header("Content-Type: ".$response['mime']);
     if ($response['cache']) {
         header('ETag: "'.md5($response['content']).'"');