Name unhandledrejection event correctly

[syvb]

Some tests in html/semantics/scripting-1/the-script-element/module do window.addEventListener("onunhandledrejection", unreachable), when they should do window.addEventListener("unhandledrejection", unreachable); - the event is called unhandledrejection, not onunhandledrejection.

This event handler should never be run when these tests pass, so this change ensures that unhandledrejection must not be fired to pass the test.