Be on par with remerkleable test coverage

[dapplion]

Proto noted that we are missing some tests from the python implementation

https://github.com/ChainSafe/ssz/blob/fe890be83a63d7c45e774f92f97f8499dc611540/test/unit/deserialize.test.ts#L39

https://github.com/protolambda/remerkleable/blob/master/remerkleable/test_impl.py