DevTools: [Documentation] Disable external XHR requests in documentation tests

LayoutTests/inspector/documentation/documentation-test.js

+function initialize_DocumentationTests()
+{
+
+InspectorTest.mockLoadXHR = function(urlPrefix, responseText)
+{
+    var originalLoadXHR = window.loadXHR;
+    window.loadXHR = function(url)
+    {
+
+        if (url.startsWith(urlPrefix))
+            return new Promise(load);
+
+        return originalLoadXHR(url);
+
+        function load(successCallback)
+        {
+            successCallback(responseText);
+            return;
+        }
+    }
+}
+
+}