Code dedup in class ImageXMLSerializationTest

Externed common parts to setUp():
- fixture with test image data
- test image
- dictionary with test images
- test serializer
These parts were equal in the most part of the methods.

bp nova-tests-code-duplication

Change-Id: I8394c7ccbd0d71ffcf5f35c64a5c307dbf579364
Merge "Code dedup in class ImageXMLSerializationTest"