Added test for CBlockIndexWorkComparator

Summary:
CBlockIndexWorkComparator is lacking tests. Required to fulfill testing requirements for D1442.

Depends on D1536.

Test Plan: make check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1555