Run snapd spread tests in CI

[valentindavid]

If doing sensitive changes in core-initrd, we need to create a draft branch in snapcore/snapd and trigger CI to run the spread tests. It would be better if the CI for core-initrd could trigger those tests as part of its CI.

[valentindavid]

The interesting tests are in:

• https://github.com/snapcore/snapd/tree/master/tests/core
• https://github.com/snapcore/snapd/tree/master/tests/nested/core