Actually test that the j-nth cell.

IPython/html/tests/util.js

@@ -394,7 +394,7 @@ casper.validate_notebook_state = function(message, mode, cell_index) {
         if (cell_index!==undefined) {
             // Is the specified cell the only cell in edit mode?
             this.test.assert(this.is_only_cell_edit(cell_index),
-                message + '; cell ' + cell_index + ' is the only cell in edit mode');
+                message + '; cell ' + cell_index + ' is the only cell in edit mode '+ this.cells_modes());
             // Is the specified cell the only cell with a focused code mirror?
             this.test.assert(this.is_cell_editor_focused(cell_index),
                 message + '; cell ' + cell_index + '\'s editor is appropriately focused');
@@ -508,11 +508,11 @@ casper.is_only_cell_edit = function(index) {
     var cells_length = this.get_cells_length();
     for (var j = 0; j < cells_length; j++) {
         if (j === index) {
-            if (!this.cell_mode_is('edit')) {
+            if (!this.cell_mode_is(j, 'edit')) {
                 return false;
             }
         } else {
-            if (this.cell_mode_is('edit')) {
+            if (this.cell_mode_is(j, 'edit')) {
                 return false;
             }
         }
@@ -539,6 +539,11 @@ casper.is_only_cell_on = function(i, on_class, off_class) {
     return true;
 };
 
+casper.cells_modes = function(){
+    return this.evaluate(function(){
+        return IPython.notebook.get_cells().map(function(x,c){return x.mode})
+    }, {});
+};
 
 casper.cell_mode_is = function(index, mode) {
     // Check if a cell is in a specific mode