Clarify source control tools cmake flag

Summary:
Rename the flag since enabling it builds no source code, it only enables
a test suite.

Depends on D7353

Test Plan:

./build-configurations.py check-source-control-tools

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7363