function getFakeExecutor(executor) {
			if (isRestorable(executor)) {
				return executor;
			}
			if (executor) {
				return fake(executor);
			}
			return fake();
		}
Returns a fake for a given function or undefined. If no function is given, a
new fake is returned. If the given function is already a fake, it is
returned as is. Otherwise the given function is wrapped in a new fake.

@param {Function} [executor] The optional executor function.
@returns {Function}