Unique statement reference is not validated

We do not seem to enforce that 'unique' statement argument actually references a leaf/leaf-list when the unique statement is introduced via "deviate add".

This was discovered by checking our behavior w.r.t a NETMOD WG query.

We have a test case which covers the following:

module foo {
    deviation "/bar:my-list" {
        deviate add {
            unique "my-leaf-a my-leaf-b";
            unique "my-leaf-c my-leaf-d";
        }
    }
}

and checks whether the unique statements are added, but alas the checks are incomplete.

The implementation interprets these arguments as "foo:my-leaf-a" and transplants them verbatim – which means that /bar:my-list has the equivalent of

unique "foo:my-leaf-a foo:my-leaf-b";
unique "foo:my-leaf-c foo:my-leaf-d";

and the model context gets happily constructed in DeviationResolutionTest.testDeviateAdd().