Additional SSZ test case

[ralexstokes]

Following up on the bug find by @Cravtos reported in https://github.com/ralexstokes/ssz_rs/issues/7

that repo runs the SSZ spec tests which suggests a deserialization test of a nested list example is missing.

in our python lib it would look something like:

l = [[]]
schema = List[List[int,1],1]
encoding = ssz.serialize(l, schema)
recovered_l = ssz.deserialize(encoding, schema)