selftests/nolibc: avoid GCC 15 -Wunterminated-string-initialization

selftests/nolibc: avoid GCC 15 -Wunterminated-string-initialization

On GCC 15 the following warnings is emitted:

nolibc-test.c: In function ‘run_stdlib’:
nolibc-test.c:1416:32: warning: initializer-string for array of ‘char’ truncates NUL terminator but destination lacks ‘nonstring’ attribute (11 chars into 10 available) [-Wunterminated-string-initialization]
 1416 |                 char buf[10] = "test123456";
      |                                ^~~~~~~~~~~~

Increase the size of buf to avoid the warning.

It would also be possible to use __attribute__((nonstring)) but that
would require some ifdeffery to work with older compilers.

Fixes: 1063649cf531 ("selftests/nolibc: Add tests for strlcat() and strlcpy()")
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250623-nolibc-nonstring-v1-1-11282204766a@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>