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

[rsimha]

Failure links: Travis, CircleCI

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 (<anonymous>)
      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 (<anonymous>)
      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.<anonymous> (/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 (<anonymous>)
	    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 (<anonymous>)
	    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.<anonymous> (/tmp/test/_init_tests.js:435:18)