[cmake] Fix json test generation
Explicitly create dir "[build directory]/src/test/data".
Right now, make check will fail for "out-of-source build" -- OUTPUT not being able to create the *.json.h files. The error occurred on my env (a box running Mac OS X), also tested on a Ubuntu instance, same result. Creating the dir solves the problem.
mkdir build && cd build cmake .. make check
Reviewers: #bitcoin_abc, schancel, deadalnix, O1 Bitcoin ABC
Reviewed By: #bitcoin_abc, schancel, deadalnix, O1 Bitcoin ABC
Subscribers: jasonbcox, teamcity
Differential Revision: https://reviews.bitcoinabc.org/D1613