Add spec tests for "ssz generic"

[ralexstokes]

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.

[ralexstokes]

the "static" tests are in ethereum/trinity#998.