Add spec tests in CI

[wemeetagain]

We need the @chainsafe/lodestar-types package to run spec tests.

A fix for this will likely look like:

• a new github action to run on pull_request
• clone lodestar monorepo, from master
• yarn links the current ssz repo
• build lodestar (if possible, just up to lodestar-config)
• run lerna run test:ssz from the lodestar monorepo