[testing] Switch to using shape type for start and size of slice

The type of both start and size of slice are tosa shape type since
0.90. Update the type of these arguments and its usage in testing.

Change-Id: Iff582982ad55d64a792183c6d46c9266510f924c
Signed-off-by: TatWai Chong <tatwai.chong@arm.com>