Fix gutils-user-database test on macOS

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

Use DYLD interposing to wrap procedures. Also use DYLD_INSERT_LIBRARIES instead of LD_PRELOAD

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