SSZ tests for light client types

[jimmygchen]

Issue Addressed

Addresses #4897

Proposed Changes

Add SSZ tests for Altair light client types. This doesn't cover forks after Altair, as Capella/Deneb forks are currently being implemented in #4896. We should change these tests from altair_only to altair_and_later once the types are implemented.

PR is built on top of this branch (#3954), the diffs for this PR is in this commit: https://github.com/sigp/lighthouse/pull/4938/commits/885958e654ff19a1742c531d4217f635141f0fc3

[jimmygchen]

Blocked by #3954 and #4896

[jimmygchen]

Included in #4946