Use new Buffer class for the entire test framework

* Replace `std::vector<uint8_t>` by `Buffer` class.
* Update `Buffer` class:
  - Add support for initial value of the buffer.
  - Always initialize the buffer with 0 by default.
* Add `pad_matrix` reference function to support extending
  the data buffer.

Signed-off-by: Viet-Hoa Do <viet-hoa.do@arm.com>

Reviewed-by: Viet-Hoa Do <viet-hoa.do@arm.com>
Reviewed-by: Anton Bondarenko <anton.bondarenko@arm.com>
Reviewed-by: Emil Ohlsson <emil.ohlsson@arm.com>
Approved-by: Anton Bondarenko <anton.bondarenko@arm.com>
Approved-by: Emil Ohlsson <emil.ohlsson@arm.com>