Add variable tensor fields for test descriptors

- Add variable_name and variable_file to the desc.json file
  for writing variable tensors to numpy
- Add the key of num_variables in the unit test declaration to
  specify the number of variable tensors in the graph

Signed-off-by: Jerry Ge <jerry.ge@arm.com>
Change-Id: I1109f66ffed52e49dbb14f4a8aca64baa2bea622