Hanging transfers regression tests

[err508]

As a sub Issue of #3124, the following is tested via #3385:

• Hanging transfers (Fix hanging transfers #3123)

  • Test the invite works on the happy path (both users online):
  • Test the invite works on the unhappy paths:
    • Both nodes open an invite at the same time
    • The node which is being invited is offline at the time of the invite
    • The node which does the invite goes offline and the invited node comes back online

• All the above is tested with different for private/private private/public public/private public/public invites

• The above scenarios work when both users are on different federated home servers

Some findings during the tests were described in #3472