refactor: univalue test cleanups
Summary:
Remove references to "downstream" from makefiles, as they are now redundant. Remove BOOST_TEST macros in favour of just using functions. Add missing call to univalue_push_throw tests.
Backport of core#25617.
Depends on D16281.
Test Plan:
ninja check-univalue
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D16282