Update output type in matmul operator

- Update eval functions of matmul operator to use acc_t,
  which in reference model captures the output type,
  consistently in intermediate operations, including subtracting the
  zero point from inputs and weights in int8.
- add unit tests to catch regressions in accumulator behaviour in
  matmul.

Signed-off-by: Eirene Vlassi Pandi <eirene.vlassipandi@arm.com>
Change-Id: If983d2f9df80e95f3ba329328f6553645c663f07