ocr-numbers: make tests more readable and allow grid('2')

I changed the test_invalid_digit code to use 'a' instead of '2' as an invalid digit. This allows coders to extend their code to produce grids for all numbers 0 - 9, without failing the tests.

The formatting of the number grids in the tests has been modified, to make the tests easier to read.
I think it would be better to remove that entire testcase.

Expecting a `ValueError` for numbers > 1 is unnecessarily limiting. But testing for non-number inputs is more of a distraction from the core of the exercise IMO.

For the other test-cases, I really like your changes!