vendor:edn has to be a build dependency of the tests

An import reads the directory at build time, so a package that dune has not
copied under the test's build dir does not resolve — and the failure is a
missing collection, not a missing file, which reads like a bug in Load.

test/dune

@@ -12,6 +12,8 @@
   (glob_files %{workspace_root}/vendor/raylib/*)
   ; The dev agent package: its Flan declarations and the C that implements them.
   (glob_files %{workspace_root}/vendor/agent/*)
+  ; The EDN tokenizer, which programs/edn.flan imports.
+  (glob_files %{workspace_root}/vendor/edn/*)
   (glob_files programs/*.flan)
   ; The reload primitive's host: a C main that dlopens what Build.shared made.
   (file reload_host.c)