Run tests against snapd stable

[valentindavid]

Otherwise we might end up merging things that depend on fixes of snapd that are not released.

[Meulengracht]

Thanks! I added a suggestion for the future in how to handle running tests with different variables, but I don't require you to change anything here.

[Meulengracht]

Thanks for doing this! Looks nice, alternative idea for future times, is that you could use a test-matrix instead of defining identical targets where only a variable is changing:

See examples:
https://github.com/snapcore/core-testing-jobs/blob/main/.github/workflows/cdimage-nightly.yml

So you could do like this:

    strategy:
      fail-fast: false
      matrix:
        channel: [stable, edge]
    with:
      snapd_branch: ${{ matrix.channel }}

Even better, maybe you could even define the templates as as matrix variable, but I'm not sure that github would accept that, as I don't know the order of evaluation when the workflow is passed (it was a hell with azure actions).

    strategy:
      fail-fast: false
      matrix:
        workflow: [tests-main.yml, tests-snapd.yml]
        channel: [stable, edge]
    uses: ./.github/workflows/${{ matrix.workflow }}
    with:
      snapd_branch: ${{ matrix.channel }}

[valentindavid]

That works, yes. We should do that in another PR. I am not totally yet convince we require a matrix here.

[alfonsosanchezbeato]

This needs rebasing