Move NodeContext from TestingSetup to BasicTestingSetup

Summary:
This will allow for the scheduler to be destructed even for tests that
only use the BasicTestingSetup. This fixes a TSAN lock order inversion
issue when running the reverselock_tests and the scheduler_tests
together.

Extracted from core PR18571.

Test Plan:

./src/test/test_bitcoin -t reverselock_tests,scheduler_tests
ninja check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6766