TST: Update test of xmode toggle: there are 4 modes.

IPython/core/tests/test_magic.py

@@ -473,7 +473,7 b' def test_cd_force_quiet():'
 def test_xmode():
     # Calling xmode three times should be a no-op
     xmode = _ip.InteractiveTB.mode
-    for i in range(3):
+    for i in range(4):
         _ip.magic("xmode")
     nt.assert_equal(_ip.InteractiveTB.mode, xmode)