Incorrect Workbox#update() test logic

[philipwalton]

Library Affected:
workbox-window

While making some updates to the Workbox tests, I noticed that the test added in #2136 for update() do not actually test the update behavior (I missed this in the PR review):

workbox/test/workbox-window/window/test-Workbox.mjs

Lines 292 to 301 in ae80e39

	it(`updates service worker and resolves with the new registration`, async function() {
	const controllerBeforeTest = navigator.serviceWorker.controller;
	const scriptURL = controllerBeforeTest.scriptURL;
	const wb = new Workbox(scriptURL);
	const reg = await wb.register();
	const newReg = await wb.update();
	expect(reg).not.to.equal(newReg);
	});

There should only ever be one registration instance per scope, so this test shouldn't be asserting calling update() returns a new object. It's only passing here because update() is returning Promise<void> rather than a registration.