Fix for mismatched extern definition in wallet test

Summary:

The extern declarations are using a raw pointer which doesn't match the
source variable's std::unqiue_ptr.

Backport of core PR11713
https://github.com/bitcoin/bitcoin/pull/11713/files

Completes T660

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3097