Saw with 157350993.1 in T324362: Adyen iframe config error messages Q2 2022: the sequel

error-20221231.gz:Dec 30 20:07:56 payments1005 adyen_gateway: 157350993:157350993.1 Client side error: Array ( [message] => ReferenceError: AdyenCheckout is not defined

Resulted in

Thoughts from discussing with ejegg:
it could be load order - let's make sure our script that tries to use it only runs after that script is loaded
I don't see us using mw.loader.load() anywhere in adyen.js
it could possibly be a load order issue!
so we call new AdyenCheckout() in some code that runs on page ready
and I've forgotten whether pageReady is before or after all the other JS loads
SO says nope: https://stackoverflow.com/questions/16983774/is-document-ready-called-after-loading-all-script-files-in-the-body
so yeah, we should do something about that!