Update staggered start delay thread.js (#172)

We are facing random issues (roughly 2/10 runs) where the second thread's command is triggered but doesn't execute any tests.

lib/thread.js

@@ -79,7 +79,7 @@ async function executeThread(thread, index) {
   const commandArguments = createCommandArguments(thread);
 
   // staggered start (when executed in container with xvfb ends up having a race condition causing intermittent failures)
-  await sleep(index * 2000);
+  await sleep((index +1) * 2000);
 
   const timeMap = new Map();