Fix scheduler test race due to BOOST_CHECK in multithreaded context

Summary:
BOOST_CHECK is not thread safe and will make TSAN to raise a false
race condition in scheduler_tests.

Test Plan:
With TSAN enabled:

./src/test/test_bitcoin -t scheduler_tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3609