[avalanche_tests] fix block index accesses

Summary: In preparation for D5238: these should be locked, and we don't need [] operator insertion so let's use the usual lookup function.

Test Plan:

CC=clang CXX=clang++ cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug
ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5237