It doesn't make much sense to test a function without understanding it first.
To locate the source code:
Right-click-hold the mouse over the while statement you have just inspected and select Edit Source
The objective of the code_int function is to place a given integer at the end of a buffer with the following format:
I[length of number][lowest order digit]....[highest order digit]
Thus the number 1234 would be stored at the end of the buffer as I44321.
That's about it. Have a look at the code you're about to test if you wish. Once ready, proceed to the next step in which you will build your test.