[gfm mode] Fix broken test setup

mode/gfm/test.js

@@ -2,9 +2,10 @@
 // Distributed under an MIT license: http://codemirror.net/LICENSE
 
 (function() {
-  var mode = CodeMirror.getMode({tabSize: 4}, "gfm");
+  var config = {tabSize: 4, indentUnit: 2}
+  var mode = CodeMirror.getMode(config, "gfm");
   function MT(name) { test.mode(name, mode, Array.prototype.slice.call(arguments, 1)); }
-  var modeHighlightFormatting = CodeMirror.getMode({tabSize: 4}, {name: "gfm", highlightFormatting: true});
+  var modeHighlightFormatting = CodeMirror.getMode(config, {name: "gfm", highlightFormatting: true});
   function FT(name) { test.mode(name, modeHighlightFormatting, Array.prototype.slice.call(arguments, 1)); }
 
   FT("codeBackticks",