Add spec tests for "ssz generic"

### What is wrong?

There are two types of spec tests we aren't running: "ssz static" and "ssz generic".

I don't expect any surprises because afaik these tests are generated w/ `py-ssz` which we directly use. We should add them to our fixture testing though just for completeness.

### How can it be fixed

Add the tests. Add the test type and test handler to parse/run the test data.