Fix the util_tests.cpp build on OSX

Summary: The osx compiler is confused with the Amount multiplication operator. Should it be int or int64_t ? Let's hint the compiler with a good'ol cast.

Test Plan:
On OSX:

ninja check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11362