Update accumulator types in conv operators

- Update eval functions of convolution operators to use acc_t
  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
  CONV2D.

Signed-off-by: Eirene Vlassi Pandi <eirene.vlassipandi@arm.com>
Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: Idb867ab10d55f04aeedfce5f8ec8c6d8fd2646d8