Fixed casting issue in coins_tests.cpp for older versions of gcc (~5.4.0)
Summary: Discovered this issue when attempting to compile using gcc 5.4.0 on Ubuntu 16.04. This issue doesn't appear to exist in newer versions of gcc (7.2.0).
Test Plan: Both cmake -GNinja .. && ninja and make check succeed.
Reviewers: deadalnix, schancel, #bitcoin_abc
Reviewed By: schancel, #bitcoin_abc
Subscribers: teamcity
Differential Revision: https://reviews.bitcoinabc.org/D1113