Decoupling PortletDataContextReferencesTest from Bookmarks

Hey Miguel,

I've finished a test, I have a couple of comments only. I've added some failsafe code for staging - null checks mainly.

The other big change is since this test was serializing the bookmarks entities I had to create two separate classes for the mock classes instead of using inner private classes. The reason is that during serializing the test class itself would be serialized, and it was causing issues during this test, but also it could case issues in the future as well, this is a bit safer I think.

Otherwise I've removed the bookmarks references from the test, and I've renamed a couple of variables.

Thanks,

Máté