Fix bonus test

[ykoh42]

Hello 👋 !

First of all, Thank you for providing the best GNL_tester ever. Thanks to this, I would pass the GNL.

For future learner, I fixed some bonus test.

There is a following bonus rule in subject.

Bonuses must be in a different file _bonus.{c/h}.

So, I fixed some files to be able to test not only get_next_line.c, get_next_line.h, get_next_line_utils.c, but also get_next_line_bonus.c, get_next_line_bonus.h, get_next_line_utils_bonus.c.

Tnx again😀 !