tests: pull in basic container tests to cover uint8

[karalabe]

This PR is a test for https://github.com/karalabe/ssz/pull/10, using the spec tests repo for it.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 33.48%. Comparing base (e1d852e) to head (f4f8d91).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11 +/- ## ========================================== + Coverage 32.06% 33.48% +1.41% ========================================== Files 57 59 +2 Lines 2919 3181 +262 ========================================== + Hits 936 1065 +129 - Misses 1940 2073 +133 Partials 43 43 ``` | [Components](https://app.codecov.io/gh/karalabe/ssz/pull/11/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P%C3%A9ter+Szil%C3%A1gyi) | Coverage Δ | | |---|---|---| | [ssz](https://app.codecov.io/gh/karalabe/ssz/pull/11/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P%C3%A9ter+Szil%C3%A1gyi) | `18.12% <ø> (ø)` | | | [sszgen](https://app.codecov.io/gh/karalabe/ssz/pull/11/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P%C3%A9ter+Szil%C3%A1gyi) | `0.00% <ø> (ø)` | |