Added new test for grade school exercise

[ThGnommy]

Hello!

After finishing the exercise grade-school, I noticed that a test is missing when looking at the instructions. So, I added it.

From the instructions:

Note that all our students only have one name (It's a small town, what do you want?) and each student cannot be added more than once to a grade or the roster. In fact, when a test attempts to add the same student more than once, your implementation should indicate that this is incorrect.

Please tell me if I missed something or if there is room for improvement.

Thank you!

[github-actions]

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