Draft: tests/gutils-user-database: do not hardcode getpwuid-preload's library name

Use Meson's build_tgt.full_path() and import('fs').name()

DYLD_INSERT_LIBRARIES should be used instead of LD_PRELOAD on macOS. Also set DYLD_FORCE_FLAT_NAMESPACE=1

See:

• https://stackoverflow.com/questions/8514783/what-is-the-exact-equivalent-to-ld-preload-on-osx
• https://stackoverflow.com/questions/34114587/dyld-library-path-dyld-insert-libraries-not-working