Replace fs::absolute calls with AbsPathJoin calls

Summary:
This adds better test coverage and will make it easier in core#20744 to remove our dependency on the two-argument boost::filesystem::absolute() function which does not have a direct equivalent in C++17.

This is a backport of core#20932 [2/2]
https://github.com/bitcoin/bitcoin/pull/20932/commits/da9caa1cedd69702aea44cb44b2fd0a2d6d56916

Depends on D11970

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11971