Add test to verify that document.currentScript.src should not get clobbered by a DOM element with name='currentScript'.

[juj]

Add test to verify that document.currentScript.src should not get clobbered by a DOM element with name='currentScript'. See https://github.com/whatwg/html/issues/10687 for details.

This is still pending browser vendor discussion, adding a test early to illustrate what it would look like.