test: Unit tests for block index filter.

Summary:
This is a partial backport of Core PR14121 : https://github.com/bitcoin/bitcoin/pull/14121/commits/6bcf0998c0b4111cadb9f0d58454266d41fd758b

Depends on D6339

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6340