Added block index test for isValid
Added block index test for isValid

Summary:
Continues work on T355.

Depends on D1559.

Test Plan: make check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1560

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