test: Fix bug in blockfilter_index_tests.

Summary:
The test case tests a chain reorganization, however the two
chains were generated in the same manner and thus produced
the same blocks.

This is a backport of Core PR17140

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6520