Adopt github/workflows to run tests on supported platforms

[SoloJacobs]

We build AM with

      - uses: ./.github/promci/actions/build
        with:
          promu_opts: "-p linux/amd64 -p windows/amd64 -p linux/arm64 -p darwin/amd64 -p darwin/arm64 -p linux/386"

but only test it with:

  test:
    name: Test
    runs-on: ubuntu-latest