Add test cases for CBlockIndex class

Summary:
New cases coverage:

• GetBlockHash()
• ToString()
• BuildSkip()
• GetAncestor()

Test Plan:
./src/test/test_bitcoin --log_level=all run_test=blockindex_tests

Suggested reviewers:

• jasonbcox
• deadalnix

Completes T355

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: schancel, jasonbcox, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1934