GCC 11 generated the warnings because the parameter `ret_buf`
was declared as `const char[10]`, but some of the arguments
provided in `run_test_snprintf` are shorter literals, like "".
Now the type of `ret_buf` is `const char *`.
Both implementations of `test_snprintf` were fixed.
Signed-off-by: Rodrigo Dias Correa <rodrigo@correas.us>