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.

Comienza localizando el comando nbdev_test y su opción do_print existente. Rastrea cómo se ejecutan las celdas del notebook y, a continuación, haz que la salida de -v identifique la celda que se está ejecutando actualmente y verifica el comportamiento con las comprobaciones existentes de nbdev_test.

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