Set result_emulated to a value that is known to be wrong in intmath_tests.cpp

Summary: Computation is done using unsigned, so that overflow is not UB.

Test Plan:

ninja check

Also do it using ubsan

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10350