Previously the meson test case would only test boost-python on linux.
With the #7909 it is now possible to use boost-python on macOS/homebrew.
This enables the boost-python test on both linux and macOS.
It also uses python.extension_module() instead of shared_library to make the
python extension module.
Set MESON_CI_JOBNAME for all CI jobs which run project tests.
(Note that ${{ github.job }} is the literal job.id used in the yaml, not
any name given to the job with job.id.name, and so is the same for all
matrix entries, and thus not suitable for our purposes there).