tests/gnrc_netif: Fix -Wformat-overflow

Fixes the following error with GCC 7

main.c: In function 'test_netif_get_name':
main.c:1054:23: error: 'sprintf' may write a terminating nul past the end of the destination [-Werror=format-overflow=]
     sprintf(exp_name, "if%d", (int)netif);
                       ^~~~~~
main.c:1054:5: note: 'sprintf' output between 4 and 9 bytes into a destination of size 8
     sprintf(exp_name, "if%d", (int)netif);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~