Fix GCC warning about `test_snprintf`

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>
diff --git a/tests/suites/host_test.function b/tests/suites/host_test.function
index db53e97..872a3a4 100644
--- a/tests/suites/host_test.function
+++ b/tests/suites/host_test.function
@@ -335,7 +335,7 @@
 #if defined(__GNUC__)
 __attribute__((__noinline__))
 #endif
-static int test_snprintf( size_t n, const char ref_buf[10], int ref_ret )
+static int test_snprintf( size_t n, const char *ref_buf, int ref_ret )
 {
     int ret;
     char buf[10] = "xxxxxxxxx";