Fix a race in validationinterface_tests

Summary: Since the block_finalized callback now runs in the scheduler thread, the test is racy as there is concurrent access to the test variables. This diff simplify the test and fixes the race.

Test Plan:
With TSAN:

ninja check-bitcoin

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15199