Add verbose option to nbdev_test

Right now `nbdev_test` has the `do_print` option but it's not enough.

Sometimes a notebook it's taking too long to be executed and I would like to know which cells are making the tests run slow. This will be particularly useful to debug why CI it's taking to long.

My suggestion it's to add a `-v` flag that outputs the cell that `nbdev_test` ran and it's running. Timing of execution per cell would be awesome but not required.