Added test for postMessage structured clones

Implemented the test discussed in Issue #388.

The name `postmessagestructuredclones` is terribly long. Other name suggestions?

Noticed the can-i-use test forces partial support to boolean true. So to test this with can-i-use I have to modify the `testify` function to handle partial support => false for only this test. But then I noticed the caniuse have some errors in their browser-support, so probably should contact Alexis to fix that first anyways...