[cmake] Fix json test generation

Summary:
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.

Test Plan:

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