[CMAKE] Move the functional tests junit reports to the test/junit dir

Summary:
The goal is to have a single output directory for all the junit reports.
The report directory is now always created when any test target is run.

Depends on D7002.

Test Plan:

ninja check-functional

Check the report is generated to the test/junit directory.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7004