[CMAKE] Add a check rule for the buildbot test

Summary:
This makes it easier to run the buidbot tests.
Because it is not part of the node software, the check-buildbot target
is not a dependency of the global check targets such as check-all.

Test Plan:

ninja check-buildbot

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7408