Fix test Jamfile.v2 to be location independent.

[grafikrobot]

In order to make the library transportable, specifically so we can support modular Boost distribution, we need to avoid encoding absolute paths in the monolithic Boost structure. This changes the test source reference to all be local to the project.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2eda741) 79.20% compared to head (ff05a1e) 79.20%.

Additional details and impacted files

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[grafikrobot]

FYI.. https://lists.boost.org/Archives/boost/2024/01/255704.php