Tests: Fix the jQuery.parseXML error reporting test

Changes:
* Remove incorrect `QUnit.testUnlessIE` usage as that util is only available
  on `master`, not here.
* Change `firstCall.lastArg` to `firstCall.args[ 0 ]` as the former API is not
  available in older Sinon versions.

test/unit/core.js

@@ -1432,15 +1432,17 @@ QUnit.test( "jQuery.parseXML", function( assert ) {
 // Support: IE 11+
 // IE throws an error when parsing invalid XML instead of reporting the error
 // in a `parsererror` element, skip the test there.
-QUnit.testUnlessIE( "jQuery.parseXML - error reporting", function( assert ) {
+QUnit[
+ document.documentMode ? "skip" : "test"
+]( "jQuery.parseXML - error reporting", function( assert ) {
  assert.expect( 2 );
 
  var errorArg, lineMatch, line, columnMatch, column;
 
  sinon.stub( jQuery, "error" );
 
  jQuery.parseXML( "<p>Not a <<b>well-formed</b> xml string</p>" );
- errorArg = jQuery.error.firstCall.lastArg.toLowerCase();
+ errorArg = jQuery.error.firstCall.args[ 0 ].toLowerCase();
  console.log( "errorArg", errorArg );
 
  lineMatch = errorArg.match( /line\s*(?:number)?\s*(\d+)/ );