Add test coverage to ensure peers without the snapshot block in their chain can be used for block downloads after background validation completes. The test fails without the fix in the previous commit.
After assumeutxo background validation completes, allow block downloads from peers that don't have the snapshot block in their best chain.
Previously, these peers were skipped until restart because `m_chainman.CurrentChainstate().SnapshotBase()` continued returning non-null even after validation finished. Add `m_chainman.CurrentChainstate().m_assumeutxo == Assumeutxo::UNVALIDATED` check to only apply the restriction while background validation is ongoing.
stringintech