IVGCVSW-3992 Add serialization support for ComparisonLayer

IVGCVSW-3992 Add serialization support for ComparisonLayer

* Added serialization support and serialization-deserialization test
  for Comparison
* Added backward compatibility tests for Equal and Greater, to make
  sure they are serialized and deserialized as Comparison
* Refactored serialization tests and reduced code duplication by
  taking advantage of operator==() recently added for Descriptors

Signed-off-by: Aron Virginas-Tar <Aron.Virginas-Tar@arm.com>
Change-Id: Id0fd94ef4b17e4e51c8005e585ea3c47f9c1bd8d