Run golden tests for some other repos as part of CI in abc repo

There are some important templates outside of the abc repo that we should be careful not to break. I'm thinking about:

- abcxyz/gcp-org-terraform-template
- abcxyz/flutter-demo
- ‎abcxyz/github-metrics-aggregator‎
- ‎abcxyz/infra-gcp‎
- ‎abcxyz/jvs‎
- ...anything else that has a spec.yaml

In the CI process for the abc repo (lint-test.yml), let's checkout these repos and run all their golden tests. This will help detect regression in the abc CLI before they're shipped.

Some ideas for how this could look:

- We could write a shell script that does a `git checkout` of each of the above repos then runs `abc templates render golden-test verify` for each one. We should wait until #311 is implemented so we can easily run every golden test in the repo.
- Extra credit: do each repo concurrently so we're not waiting for a long time.

We'll also need to actually *create* golden tests for the above repos using `abc templates render new-test`.

Tentatively assigning to @capri-xiyue who's working on golden tests.

cc @verbanicm who has an interest in some of the above repos