[cmake] move json test generation script out of cmake

Summary:
As per title. Previous version used GNU sed specific
features and was broken on MacOS X.

Test Plan: cmake && ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1349