Add integration test covering upgrades

[joshtrichards]

• Major upgrades: N.0.8 -> N+1.0.0 (e.g. 28.0.8 -> 29.0.0)
• Minor upgrades: N.0.1 -> N.0.2 (e.g. 29.0.1 -> 29.0.2)

Ideally some infrastructure variations:

• standard apache/mod_php CLI-based update/upgrade
• standard apache/mod_php GUI-based update/upgrade
• FPM apache (or nginx)/php-fpm CLI-based update/upgrade

[provokateurin]

While triaging issues a few weeks back it seemed that upgrading usually doesn't cause problems (at least I didn't find many tickets about it), but it makes sense to have it covered just in case 👍
I want to first write more unit tests for the existing code and then we can come around to tackle this.