[cmake] link test runners by default

Summary: This is not an expensive operation, is properly tracked by dependencies and ensure the presence of these file is predictable.

Test Plan:

rm test/functional/test_runner.py
ninja

Check test/functional/test_runner.py was linked.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4607