Move NULLPTR macro so that it can be used by all unit tests

Summary: Rather than copy-paste this to another test that I'm working on, it should be available to the whole unit test suite.

Test Plan: ninja check + CI diff should build (it uses an older version of Boost)

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2500