libtest/stub_gssapi: use "real" snprintf

... since it doesn't link with libcurl.

Reverts the commit dcd6f810 changes from this file.

Bug: https://curl.haxx.se/mail/lib-2019-01/0000.html
Reported-by: Shlomi Fish
Reviewed-by: Daniel Gustafsson
Reviewed-by: Kamil Dudka

Closes #3434