Fix WebView tests with WebKitGTK 2.43+

It seems that WebKitGTK is now failing to navigate to about: URLs unless
they are about:blank or about:srcdoc, so use about:srcdoc as the
alternate URL to fix the WebView tests.

Ref: 3c3163e71f

tests/controls/webtest.cpp

@@ -45,7 +45,11 @@ public:
         }
         else
 #endif
+#if wxUSE_WEBVIEW_WEBKIT2
+            m_alternateHistoryURL = "about:srcdoc";
+#else
             m_alternateHistoryURL = "about:";
+#endif
     }
 
     ~WebViewTestCase()