Try to make #changeToNewRole a bit more reliable

[brbrr]

There are 2 buttons with the same locators in the EditTeamMemberPage. 'Update' button is in disabled state it role was not changed.

This PR makes #changeToNewRole click only on active 'Update' button, also it chains the driver-related calls to guarantee synchronous execution

[alisterscott]

Is this actually any different than what was there before?

I don't really understand why you'd need to chain these actions here?

[brbrr]

It is different :)

So while I tried to debug CircleCI failures I noticed that Invites test fails after that method call (https://github.com/Automattic/wp-e2e-tests/blob/master/specs/wp-invite-users-spec.js#L706)
Not sure why, but the local run was clicking on the disabled button(before it selects 'Author' from dropdown)

So here I changed button locator since there 2 buttons located with button.form-button.is-primary and it should click only not disabled button.

I don't really understand why you'd need to chain these actions here?

In my understanding, all(or most) of the driver and driverHelper methods return a promise. And in my experience, WebDriverJS promise manager isn't reliable enough, that's why I prefer to chain all these promise calls whenever it's needed or not. As a side benefit - it would be much easier to convert then chains into await calls when we'll move to async/await style (#329)

[brbrr]

Its not needed