IVGCVSW-3991 Make Descriptor objects comparable and refactor LayerVisitor tests

IVGCVSW-3991 Make Descriptor objects comparable and refactor LayerVisitor tests

* Implemented operator==() for Descriptor structs
* Refactored TestNameAndDescriptorLayerVisitor to eliminate code duplication
  by using templates and taking advantage of the fact that descriptor objects
  can now all be compared the same way using ==
* Cleaned up TestNameOnlylayerVisitor by moving all test cases for layers
  that require a descriptor to TestNameAndDescriptorLayerVisitor

Signed-off-by: Aron Virginas-Tar <Aron.Virginas-Tar@arm.com>
Change-Id: Iee38b04d68d34a5f4ec7e5790de39ecb7ab0fb80