Fix invalid augmentations

Fix invalid augmentations

choice/case structures are collapsed to their parent data tree
node, hence it is invalid to define (in same namespace) the equivalent
of:

container {
  leaf foo { type string; }
  choice some {
    case foo {
      leaf foo { type string; }
    }
  }
}

Fix up the testing model.

Change-Id: I1e3ef8f9b38016e0290221f22b8233fba176bc0e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>