Add backend tests to bundled extensions

[askvortsov1]

• Start adding integration tests to bundled extensions
  • flarum/tags (7122861)
  • flarum/subscriptions (fix(subscriptions): reply notifications not working #3445)
  • flarum/mentions (Add integration tests mentions#66)
  • flarum/flags (be59e77)
  • flarum/approval (fix(approval): unapproved posts visible to all when no visibility scopers are added #3510)
  • flarum/sticky (test(sticky): list discussions works as expected with stickies #3507)
  • flarum/suspend (test(suspend): test that suspension works as expected #3511)
  • flarum/likes (test(likes): like action behavior #3512)

I'll use the update feature of the extension generator for this, then fill in some basic tests.

[SychO9]

Completed 🫡