[RCU] Use assert in tests instead of BOOST_CHECK when they aren't in the main thread.

Summary: Boost test is unable to manage these properly, so asserts are preferable.

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2365