Avoid spurious boost output in scheduler_tests

Summary:
Boost test macros try to output some message, which leads to a messy
debug output when called in intricated threads.
Use assert in these cases solves the issue.

Backport of Core PR14069
https://github.com/bitcoin/bitcoin/pull/14069/files

Test Plan:

./src/test/test_bitcoin -t scheduler_tests

Without the patch it will sometime output partial boost debug info.
With the patch the output will remain clean.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3232