Fix memory leak in work_comparator_tests.cpp

Summary: The CBlockIndex are never freed.

Test Plan:
Build with asan, then:

./src/test/test_bitcoin -t work_comparator_tests

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3568