ci: run Gossamer related polkadot-tests on pull requests

## Task summary

We should run the Gossamer related [polkadot-tests](https://github.com/w3f/polkadot-tests) on each pull request to ensure conformance and integration tests are passing before merging.

They have 2 workflows:

- [conformance.yml](https://github.com/w3f/polkadot-tests/blob/654aa5ecc5fef657f2ca63c31a2f88a191ea284a/.github/workflows/conformance.yml)
- [integration.yml](https://github.com/w3f/polkadot-tests/blob/654aa5ecc5fef657f2ca63c31a2f88a191ea284a/.github/workflows/integration.yml)

We should adapt these two to be focused on Substrate (reference) + Gossamer, and to be run in this repository.

Note they use git submodules, in our case we can probably just use a tagged substrate release which we can update manually.
We could ideally also use it with Docker images to have substrate pre-packaged.