Improve WSL 2 support

### What is the problem or limitation you are having?

Hello,

Under this issue I would like to track PRs improving WSL support in Toga.
Right now, after this simple PR https://github.com/beeware/toga/pull/4572 Toga tests are runnable but they fail.

#### GTK 3
```python
FAILED tests/app/test_desktop.py::test_current_window
FAILED tests/widgets/canvas/test_canvas.py::test_reset_transform
FAILED tests/window/test_window.py::test_focus_events[Window-second_window_kwargs0]
```

#### GTK 4
```python
=========================== short test summary info ============================
FAILED tests/app/test_desktop.py::test_current_window
FAILED tests/widgets/canvas/test_canvas.py::test_reset_transform
FAILED tests/widgets/test_button.py::test_font
FAILED tests/widgets/test_button.py::test_focus
FAILED tests/widgets/test_label.py::test_font
FAILED tests/widgets/test_textinput.py::test_font
FAILED tests/window/test_window.py::test_visibility[Window-second_window_kwargs0]
```

I'm working on it.

### Describe the solution you'd like

I think to find a ultimate WSL/WSLg setup to makes all Toga tests passes.

### Describe alternatives you've considered

We can just ignore existence of WSL 2 and do not support it.

### Additional context

_No response_

Start with PR #4572 and run the listed GTK 3 and GTK 4 failures under the WSL/WSLg setup. Inspect tests/app/test_desktop.py, tests/widgets/canvas/test_canvas.py, the button, label, and textinput tests, and tests/window/test_window.py; done means the reported Toga tests pass in both GTK versions.

Written by the indexing model from the issue text.