Add additional tests to clock exercise for extreme I64 values, fixes #150

[ageron]

This PR adds tests that ensure that the implementation can handle the min and max I64 values.
This makes the exercise a bit harder, perhaps we should bump its difficulty to 3.

Another option would be to let the functions return Result Str _ rather than Str, and just ensure that the tests for the min/max I64 values don't crash: we can simply make sure that the result for these test cases are either Ok with the correct value, or any Err. Wdyt?

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

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[isaacvando]

Leaving the approach as it is now and bumping it to 3 sounds good to me 👍
If it is possible to reasonably avoid returning a result I always like to do so.