Testing support for MUL with shift as input

Always create the shift as a tensor for all types in testing.
In the reference model, set the shift operand to be available for
all types, but only read in the shift tensor for i32.

Signed-off-by: Jeremy Johnson <jeremy.johnson@arm.com>
Signed-off-by: TatWai Chong <tatwai.chong@arm.com>
Change-Id: Ia267cbf8b63ca0a9c97b38e8fb4db83eeb8c0538