Add Windows and macOS GUI tests to GitHub Actions

[drew2a]

This PR closes #7086, closes #7087

To my great surprise, it worked out of the box.

The only one issue was "how to call a workflow with the custom matrix?".
The solution is to use fromJson for passing the matrix as a JSON object:

name: PR

on:
  pull_request:
    types:
      - opened
      - synchronize
      - ready_for_review

jobs:
  magic:
    uses: ./.github/workflows/magic.yml
    with:
      matrix: '{"os":["macos-latest","ubuntu-latest"]}'}

name: Magic

on:
  workflow_call:
      matrix:
        default: '{"os":["windows-latest"]}'
        type: string
        required: false

jobs:
  run:
    runs-on: ${{ matrix.os }}
    strategy:
      matrix: ${{fromJson(inputs.matrix)}}

Refs:

• https://stackoverflow.com/questions/57946173/github-actions-run-step-on-specific-os
• https://github.com/pyvista/setup-headless-display-action
• https://docs.github.com/en/enterprise-cloud@latest/actions/learn-github-actions/expressions#example-returning-a-json-object
• https://stackoverflow.com/questions/59977364/github-actions-how-use-strategy-matrix-with-script