Fix flaky tests in `PopupMenuComponent`

[smbea]

What should we do?

Some PopupMenuComponentSpec test are flaky (see here). This is due to the async rendering.

Currently, a setTimeout is being used but it's is still causing flakiness and we should not rely on this.

Why should we do it?

This affects our CI and is annoying.

[smbea]

Some initial considerations to handle this:

• use preact's utilityact like we use in properties panel
  • not exported by preact direcly, its export in preact/test-utils
  • we would have to include it in diagram-js-ui
• implement something that relies on requestAnimationFrame like act: https://github.com/preactjs/preact/blob/master/test-utils/src/index.js#L27
• fire popupMenu.closed, popupMenu.opened, etc and rely on popupMenu.on...