New test for ft_substr

[Tesla-J]

This new test helps to check if allocated memory is excessive when we start close to the end of the string and len is less than the length of the string.

Previous tests do not take this in account, mbueno-g's test helped me to find this out.

[Tripouille]

Hey Tesla-J, thanks for your interest. Isn't that undefined behavior since the start is greater than the end? If so, it shouldn't be tested.