Added CMake function to detect BOOST_TEST_DYN_LINK

Summary:
Avoid code duplication by making BOOST_TEST_DYN_LINK detection into a CMake
module.

Test Plan:

ninja check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4432