With :peer, which replaced the now-deprecated :slave:

opts = %{name: ~c"foo", host: ~c"localhost", connection: :standard_io}
{:ok, peer_pid, _name} = :peer.start_link(opts)

:peer.call(peer_pid, :code, :add_paths, [:code.get_path()])
:peer.call(peer_pid, IO, :puts, ["test"])
# "test"

Problem there is that slave node was spawned without any extra libraries outside the Erlang standard distribution. That mean that module IO (Elixir.IO to be exact) do not exists on that module. With save module started like this you can use only Erlang standard library, so to achieve what you want you need to use fn -> :io.format("test") end.

Thanks for the help!
I had a look into some of the code, and was able to find the correct commands which let me load the code path into the slave:

:os.cmd('epmd -daemon')
Node.start(:master@localhost, :shortnames)
:slave.start(:localhost, 'slave')
:rpc.block_call(:test@localhost, :code, :add_paths, [:code.get_path()])
...
Node.spawn(:slave@localhost, fn -> IO.puts "test" end)
...
#PID<10741.85.0>
iex(master@localhost)9> test

It worked!