Merge bitcoin/bitcoin#30678: wallet: Write best block to disk before backup

f20fe33e94c6752e5d2ed92511c0bf51a10716ee test: Add basic balance coverage to wallet_assumeutxo.py (Fabian Jahr)
037b101e808ccf9e717751619e04f6e87d614efd test: Add coverage for best block locator write in wallet_backup (Fabian Jahr)
31c0df038909e40fe9618a4595254907ed1de907 wallet: migration, write best locator before unloading wallet (furszy)
7e3dbe4180cbeb65e59b53d9fa98509e9189549d wallet: Write best block to disk before backup (Fabian Jahr)

Pull request description:

  I discovered that we don't write the best block to disk when trying to explain the behavior described here: https://github.com/bitcoin/bitcoin/pull/30455#discussion_r1719951882

  In the context of that test, the behavior is confusing and I think it also shows that one of the already existing tests in `wallet_assumeutxo.py` doesn't actually test what it says. It only fails because the best block isn't written and actually, the height of the backup that is loaded is at the snapshot height during backup. So it really shouldn't fail since it's past the background sync blocks already.

  I'm not sure if this is super relevant in practice though so I am first looking for concept ACKs on the `BackupWallet` code change. Either way, I think this behavior should be documented better if it is left as is and the test should be changed.

ACKs for top commit:
  achow101:
    ACK f20fe33e94c6752e5d2ed92511c0bf51a10716ee
  furszy:
    ACK f20fe33

Tree-SHA512: bb384a940df5c942fffe2eb06314ade4fc5d9b924012bfef3b1c456c4182a30825d1e137d8ae561d93d3a8a2f4d1c1ffe568132d20fa7d04844f1e289ab4a28b

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