[Credentialless] Fix flakes about iframeTest.js

In the various tests, there are up to 8 <iframe> navigating concurrently.

There are no ways to detect an <iframe> fails to load other than using
a timeout. See whatwg/html#125.

The 3000ms timeout was sometimes too short. This cause flakes.
To reduce the rate of false positive, the timeout has been increased to
6000ms.

R:titouan@chromium.org
CC:lyf@chromium.org
Fixed:1215956
Bug:1175099
Change-Id: I7bedccb351ff62d55b6020403aac5457608e5497

Author: ArthurSonzogni

html/cross-origin-embedder-policy/credentialless/resources/iframeTest.js

@@ -61,7 +61,7 @@ const iframeTest = function(
     // There are no interoperable ways to check an iframe failed to load. So a
     // timeout is being used.
     // See https://github.com/whatwg/html/issues/125
-    step_timeout(()=>send(test_token, "block"), 3000);
+    step_timeout(()=>send(test_token, "block"), 6000);
 
     assert_equals(await receive(test_token), expectation);
   }, description);