[script_tests] add small NUM2BIN case

Summary:
Implementors may be tempted to have NUM2BIN bail out early if the
input is already of the requested size, however this is incorrect--
in the case of a "negative zero" representation, NUM2BIN must flip
the sign bit.

Test Plan: make check

Reviewers: deadalnix, Fabien, jasonbcox, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3610