Separate single environment test into its own workflow

[markotoplak]

This is a refactor which should not break anything.

If single environment tests are separated, add-ons call test specific additional environments. For example:

jobs:
  normal:
    name: Usual
    uses: biolab/orange-ci-cd/.github/workflows/test-addons.yml@master
  dask:
    name: Dask
    uses: biolab/orange-ci-cd/.github/workflows/test-single-env.yml@master
    with:
      os: ubuntu-latest
      python-version: '3.10'
      tox_env: dask
      pyqt: '5.15.*'

The above worklow runs the standard add-on tests plus an additional test with a specific tox environment (dask). See Quasars/orange-spectroscopy#702 for that PR.

[JakaKokosar]

I would rename 'test single environment' to something simpler like 'test job'. I was just confused because I didn't understand what single env referred to. It all made sense later.

I think we can merge, looks nice, thanks!

[markotoplak]

Thanks for the better name. I'll wait to see if my PR tests run and then merge.