Regular failures in `test/unit/test-iframe-helper.js` due to cross-origin frame access

Failure links: [Travis](https://travis-ci.com/github/ampproject/amphtml/jobs/473957046), [CircleCI](https://app.circleci.com/pipelines/github/ampproject/amphtml/189/workflows/0b9bc954-c5f1-42f2-b77d-72b188bbb362/jobs/1134/parallel-runs/0/steps/0-110)

```sh
IT => "after each" hook for "should set sentinel on postMessage data"
✗ Error: Blocked a frame with origin "http://localhost:9876" from accessing a cross-origin frame.
at Function.defineProperty ()
at Function.restore (/tmp/node_modules/sinon/lib/sinon/util/core/wrap-method.js:130:24)
at /tmp/node_modules/sinon/lib/sinon/sandbox.js:30:9
at Array.forEach ()
at applyOnEach (/tmp/node_modules/sinon/lib/sinon/sandbox.js:29:5)
at Sandbox.restore (/tmp/node_modules/sinon/lib/sinon/sandbox.js:166:9)
at Context. (/tmp/test/_init_tests.js:435:18)
Chrome Headless 88.0.4324.96 (Linux x86_64) "after each" hook for "should set sentinel on postMessage data" FAILED
Error: Blocked a frame with origin "http://localhost:9876" from accessing a cross-origin frame.
at Function.defineProperty ()
at Function.restore (/tmp/node_modules/sinon/lib/sinon/util/core/wrap-method.js:130:24)
at /tmp/node_modules/sinon/lib/sinon/sandbox.js:30:9
at Array.forEach ()
at applyOnEach (/tmp/node_modules/sinon/lib/sinon/sandbox.js:29:5)
at Sandbox.restore (/tmp/node_modules/sinon/lib/sinon/sandbox.js:166:9)
at Context. (/tmp/test/_init_tests.js:435:18)
```