Update browser/component-tests to use `await raf()`

[nojvek]

Currently we use done() and callbacks but async would be better style

      window.requestAnimationFrame(() => {
        childEl = el.getElementsByTagName(`nested-child`)[0];
        done();
      });

Example: test/server/component.js

    it('applies only the last state in a given frame', async function() {
       ...
      expect(domPatcher.el.textContent).to.eql('Value of foo: bar');
      await raf();
      expect(domPatcher.el.textContent).to.eql('Value of foo: whew');
    });

[tdumitrescu]

Yup, see https://github.com/mixpanel/panel/blob/2c1ff8f1f99cb873aca1d49aa20f9e08c760f7f7/test/server/component.js for existing usage examples from the server-side tests that we can also use client-side now that browser test modules are transpiled for all supported browsers.