This is so we can create blocks within unittests and have them
be processed by specific chainstates (instead of the just the
active one).
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/21526 | core#21526]] [11/12]
https://github.com/bitcoin/bitcoin/pull/21526/commits/2705570109a2a90ecfd3f4180944498626fc2707#