Add test for Capability Delegation

[dontcallmedom]

https://wicg.github.io/capability-delegation/spec.html adds a new delegate parameter to the options argument of postMessage, taking for value a string matching a feature policy.

window.postMessage('', {delegate: 'doesnotexist'}) allows to detect whether delegation is supported at all (it throws in a browser that supports it, it doesn't otherwise)

window.postMessage('', {delegate: 'payment'}) allows to detect whether delegation for payment specifically is supported at all (it doesn't throw in a browser that supports it)