test: disable unit tests that are UB before C++20

Summary:
See https://en.cppreference.com/w/cpp/language/operator_arithmetic.html#Bitwise_shift_operators
Before C++20:

For negative a, the behavior of a << b is undefined.

Test Plan:
With Clang + UBSAN:
ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18601