test: Report InitBlockIndex result

Summary:
If InitBlockIndex fails, then it will segfault later. Same for the later
ActivateBestChain. BOOST_REQUIRE the result, so that an error will be
reported and the test case aborted.

Backport of core's PR9904

Test Plan:

make check

Reviewers: schancel, #bitcoin_abc, dagurval

Reviewed By: #bitcoin_abc, dagurval

Differential Revision: https://reviews.bitcoinabc.org/D903