fix: Fix setting up a profile and immediately transferring to a second device

[Hocuri]

Found and fixed a bug while investigating #6656. It's not the same bug, though.

Steps to reproduce this bug:

• Create a new profile
• Transfer it to a second device
• Send a message from the first device
• -> It will never arrive on the second device, instead a warning will be printed that you are using DC on multiple devices.

The bug was that the key wasn't created before the backup transfer, so that the second device then created its own key instead of using the same key as the first device.

In order to regression-test, this PR now changes clone() to use "Add second device" instead of exporting and importing a backup. Exporting and importing a backup has enough tests already.

This PR also adds an unrelated test test_selfavatar_sync().

The bug was introduced by #6574 in v1.156.0

[iequidoo]

Maybe add a Rust test instead which checks that the key is the same on devices after a backup export/import? We don't even need to check that synchronization works (if keys are different, it's a bug anyway)

[Hocuri]

Maybe add a Rust test instead

I don't think it's possible to do iroh-add-second-device in Rust? And the issue only happened with iroh-add-second-device, not with export-backup

[link2xt]

I don't think it's possible to do iroh-add-second-device in Rust? And the issue only happened with iroh-add-second-device, not with export-backup

It is possible to do everything in Rust, but better not add online tests there. We have around 5 online tests in Rust and I don't like that they fail when offline, better keep them in Python if it requires being online.

[Hocuri]

In order to regression-test, this PR now changes clone() to use "Add second device" instead of exporting and importing a backup. I checked that when I revert the fix, then 5 tests fail now. Exporting and importing a backup has enough tests already.

This PR also adds an unrelated test test_selfavatar_sync().