unit test updates

- Add XVFB (virtual display)
  - Re-activate viewer gui test

.github/workflows/pytest.yml

@@ -35,7 +35,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install flake8 pylint mypy pytest pytest-mock wheel
+          pip install flake8 pylint mypy pytest pytest-mock pytest-xvfb wheel
           pip install -r ./requirements/requirements_${{ matrix.backend }}.txt
       - name: Lint with flake8
         run: |

tests/tools/preview/viewer_test.py

@@ -116,7 +116,6 @@ class TestFacesDisplay():
         f_display.set_display_dimensions(dimensions)
         assert f_display._display_dims == dimensions
 
-    @pytest.mark.skip(reason="Headless tkinter will error")
     @pytest.mark.parametrize("columns, face_size", _PARAMS, ids=_IDS)
     def test_update_tk_image(self,
                              columns: int,
@@ -133,8 +132,6 @@ class TestFacesDisplay():
         mocker: :class:`pytest_mock.MockerFixture`
             Mocker for checking _build_faces_image method called
         """
-        # TODO find out how we can test this on a headless system
-        # Launching tk.Tk() will result in an error because a display is not found
         f_display = self.get_faces_display_instance(columns, face_size)
         f_display._build_faces_image = cast(MagicMock, mocker.MagicMock())  # type:ignore
         f_display._get_scale_size = cast(MagicMock,  # type:ignore