Tests: run npm install from dune

[jchavarri]

While publishing in opam repository, I realized calling dune runtest would fail because we weren't tracking the dependency in node_modules anywhere (it was assumed that npm install would be called manually).

This PR adds this dependency inside dune itself by leveraging the recently added directory targets. ref: ocaml/dune#6168.

The yarn.lock was removed as it would lead to permission issues:

npm ERR! Error: EACCES: permission denied, open '/.../jsoo-react/_build/default/test/yarn.lock'

As there are barely any deps and we are already constraining to minor versions of them, we should be fine.