add tests on namespace functionality

[jomiklos]

Add a/some test(s) to verify whether device can work with a "namespace" concept.
Think of and implement at least sanity (or more) checks for some device specific/configurable path.

Example:

• GetRequest path == openconfig-interfaces:interfaces
  vs
• GetRequest path == interfaces

Consider longer paths with namespace values placed on different/various spots of the path(s) being tested.