Add a plain text cell to the test notebook

tests/test.ipynb

@@ -141,6 +141,12 b''
      ]
     },
     {
+     "cell_type": "plaintext",
+     "source": [
+      "plain text"
+     ]
+    },
+    {
      "cell_type": "code",
      "collapsed": true,
      "input": [],