Bug 1125636 - Make the test browser_styleeditor_xul.js not depend on about:config's internal structure. r=jwalker

browser/devtools/styleeditor/test/browser.ini

@@ -42,6 +42,7 @@ support-files =
   test_private.html
   doc_uncached.css
   doc_uncached.html
+  doc_xulpage.xul
 
 [browser_styleeditor_autocomplete.js]
 [browser_styleeditor_bug_740541_iframes.js]

browser/devtools/styleeditor/test/browser_styleeditor_xul.js

@@ -6,7 +6,7 @@
 
 waitForExplicitFinish();
 
-const TEST_URL = "about:config";
+const TEST_URL = TEST_BASE + "doc_xulpage.xul";
 
 add_task(function*() {
   let tab = yield addTab(TEST_URL);

browser/devtools/styleeditor/test/doc_xulpage.xul

@@ -0,0 +1,7 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="simple.css" type="text/css"?>
+<!DOCTYPE window>
+<window xmlns:html="http://www.w3.org/1999/xhtml"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <label value="Simple XUL document" />
+</window>