include cmath in util_tests

Summary: A missing include causes some builds to fail. It seems like cmath was previously imported indirectly via boost/thread/thread.hpp (removed from setup_common.h in D10992)

Test Plan:
ninja all check-all

@bot build-linux64

Reviewers: #bitcoin_abc, tyler-smith

Reviewed By: #bitcoin_abc, tyler-smith

Subscribers: tyler-smith

Differential Revision: https://reviews.bitcoinabc.org/D10999