I was able to get my tests to work. Timo's advice of using HttpSessionStore, led me to realize that WicketTester and, more importantly, DummyApplication (which I was not using since I'm not using JUnit) seem to take care of the mocking serialization problems.
I decided to use those built-in classes for now, until I can create a custom ones specifically for use with Specs (the testing framework i'm using). I'm not seeing the 2.7.3 serialization issue as of now, but it's good to now for future problems. By the way, I think the hardest part about Unit testing and TDD specifically is that, if you don't know your frameworks really well (I'm new to Wicket), then you constantly run into brick walls. It's kind of a chicken before the egg problem.
