idlharness.js needs to be able to exclude members

[annevk]

For HTML we want to do something like this:

// This is tested in ../webappapis/system-state-and-capabilities/the-navigator-object/navigator.any.js
idlArray.add_untested_idls(`interface Navigator {
  [Exposed=Window] boolean taintEnabled(); // constant false
  [Exposed=Window] readonly attribute DOMString oscpu;
};`);

Except in a way that works, as this doesn't.

cc @foolip