tests: Make sure QWebEnginePage is deleted early

If we use the "webengineview" fixture and access
QWebEngineProfile.defaultProfile(), it could happen that the profile
outlives the page. In that case, we get a warning on pytest shutdown:

  Release of profile requested but WebEnginePage still not deleted. Expect troubles !

Thus, let's make sure the page is cleaned up earlier.

tests/helpers/fixtures.py

@@ -444,7 +444,8 @@ def webengineview(qtbot, monkeypatch, web_tab_setup):
     monkeypatch.setattr(objects, 'backend', usertypes.Backend.QtWebEngine)
     view = QtWebEngineWidgets.QWebEngineView()
     qtbot.add_widget(view)
-    return view
+    yield view
+    view.setPage(None)  # Avoid warning if using QWebEngineProfile
 
 
 @pytest.fixture