Fix and clean-up bad EAVT parsing spec.

This tests what I think the test was getting at in a more direct way.

/cc @charleso @raronson for review.
Just to keep the tests fast. String generation is not constant time, so you want to prefer primitives in general. With string it is easy to drop into quadratic or worse behaviours (Lists of Value etc..). By reducing their frequency you will still get them and their edge cases, but you will don't have to wear it quiet as much.