test: export public symbols in addons tests

Upcoming changes to node-gyp will turn on `-fvisibility=hidden` on
macOS. Ensure that public symbols that are dlsym'd have default
visibility.

Refs: https://github.com/nodejs/node/pull/28647
Refs: https://github.com/nodejs/node-gyp/pull/1828

PR-URL: https://github.com/nodejs/node/pull/28717


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>