Edje: Fix test case to show proper use of part APIs

Edje: Fix test case to show proper use of part APIs

The one and only valid use case is calling a function directly on
the part handle, like:
  func(part_get(obj, "part"), args).

This does not change the actual proxy object lifecycle or resolve
any potential issues, but this should clear up things in terms
of scope for proper (supported) use.

efl_content_get may become efl_part() and return a temporary object,
valid for a single function call. Those are "details" still to be
refined.