test(tvix/store/protos): add go tests for Validate()

We already have validation tests for Rust, let's add the missing ones
for golang too.

Change-Id: Iaf3a3e1ee72d5647da3f2aa977d6e0d0379b2ce5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9595
Reviewed-by: Brian McGee <brian@bmcgee.ie>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nar-bridge/pkg/exporter/export.go

@@ -194,9 +194,6 @@ func Export(
 	}
 }
 
-// TODO: add validation functions to Directory in both rust and golang, to
-// validate the keys in directories, files and symlinks are sorted.
-
 // drainNextNode will drain a directory message with one of its child nodes,
 // whichever comes first alphabetically.
 func drainNextNode(d *castorev1pb.Directory) interface{} {