Refactor <webview> tag tests

[alexeykuzmin]

Rewrite tests with async/await.

[ckerr]

LGTM, but before I Approve -- If I'm reading correctly, this is not about a functional change, but about making the log output more readable. Is that correct?

[alexeykuzmin]

@ckerr In several case it does make output more readable. And that was initial reason for some refactoring.

But mostly the changes here are about making code simpler, and easier to read and change. (Tests have to be super simple.)
Promises and async/await is the best thing happened to JS ever =)

[alexeykuzmin]

@ckerr Did I answer you question? Anything else?

[alexeykuzmin]

A good example would be a test with two event listeners:

    it('loads native modules when navigation happens', (done) => {
      <...>

      const listener = () => {
        webview.removeEventListener('did-finish-load', listener)
        const listener2 = (e) => {
          assert.equal(e.message, 'function')
          done()
        }
        webview.addEventListener('console-message', listener2)
        webview.reload()
      }
      webview.addEventListener('did-finish-load', listener)
      webview.setAttribute('nodeintegration', 'on')
      webview.src = `file://${fixtures}/pages/native-module.html`
      document.body.appendChild(webview)
    })

changed to
(non-essential part is omitted <...>)

    it('loads native modules when navigation happens', async function () {
      <...>

      await loadWebView(webview, {
        nodeintegration: 'on',
        src: `file://${fixtures}/pages/native-module.html`
      })

      webview.reload()

      const {message} = await waitForEvent(webview, 'console-message')
      assert.equal(message, 'function')
    })

[ckerr]

Yes that answers my question, I was just making sure that I wasn't misunderstanding the scope of the PR.

As for the PR itself, these changed tests are a lot more straightforward / readable. Thanks for these improvements!