Update CheckBlockIndex invariants for chains based on an assumeutxo snapshot
https://github.com/bitcoin/bitcoin/pull/27746/commits/272fbc370c4e133d31d9f1d34e327cc265c5fad2
test: Clear block index flags when testing snapshots
When simulating a snapshot, remove the HAVE_DATA status for blocks below the
snapshot height, to simulate never having downloaded them at all. This makes
tests more realistic (and more closely match what will happen when using
assumeutxo).
https://github.com/bitcoin/bitcoin/pull/27746/commits/3cfc75366e6596942cbc84f354f42dfd7fc5c073
This is a partial backport of core#27746
Depends on D17433