build: header-only Boost

Summary:

This PR converts our Boost usage to header only. We switch from using our last remaining Boost lib (unit test), to using it's header-only implementation (see https://www.boost.org/doc/libs/1_78_0/libs/test/doc/html/boost_test/adv_scenarios/single_header_customizations/multiple_translation_units.html).

This is a backport of core#24301
Depends on D13990

Test Plan:
ninja all check-all
All CI cross builds
guix build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13991