Avoid test a web page needing GBM device

The original test_multi_window.py tries to load the web page rendering
with the GBM device. But, it shows the error and blocks test:

test_multi_window.py
============================= test session starts ==============================
platform linux -- Python 3.10.12, pytest-6.2.5, py-1.10.0, pluggy-0.13.0
rootdir: /home/dev/pywebview/tests
collected 5 items

test_multi_window.py ..Unable to access the GBM device, disabling DMABuf video sink.
[pywebview] Main window failed to start
Traceback (most recent call last):
  File "/home/dev/pywebview/tests/util.py", line 72, in thread
    thread_func(window)
  File "/home/dev/pywebview/tests/test_multi_window.py", line 110, in load_url
    child_window.destroy()
  File "/home/dev/pywebview/webview/window.py", line 45, in wrapper
    raise WebViewException('Main window failed to start')
webview.util.WebViewException: Main window failed to start

The graphic hardware acceleration and rendering API should be invoked by
system's WebView/WebKit library, not pywebview. So, test with simple web
pages.