selftests/tcp_ao: Printing fixes to confirm with format-security

On my new laptop with packages from nixos-unstable, gcc 12.3.0 produces
> lib/setup.c: In function ‘__test_msg’:
> lib/setup.c:20:9: error: format not a string literal and no format arguments [-Werror=format-security]
>    20 |         ksft_print_msg(buf);
>       |         ^~~~~~~~~~~~~~
> lib/setup.c: In function ‘__test_ok’:
> lib/setup.c:26:9: error: format not a string literal and no format arguments [-Werror=format-security]
>    26 |         ksft_test_result_pass(buf);
>       |         ^~~~~~~~~~~~~~~~~~~~~
> lib/setup.c: In function ‘__test_fail’:
> lib/setup.c:32:9: error: format not a string literal and no format arguments [-Werror=format-security]
>    32 |         ksft_test_result_fail(buf);
>       |         ^~~~~~~~~~~~~~~~~~~~~
> lib/setup.c: In function ‘__test_xfail’:
> lib/setup.c:38:9: error: format not a string literal and no format arguments [-Werror=format-security]
>    38 |         ksft_test_result_xfail(buf);
>       |         ^~~~~~~~~~~~~~~~~~~~~~
> lib/setup.c: In function ‘__test_error’:
> lib/setup.c:44:9: error: format not a string literal and no format arguments [-Werror=format-security]
>    44 |         ksft_test_result_error(buf);
>       |         ^~~~~~~~~~~~~~~~~~~~~~
> lib/setup.c: In function ‘__test_skip’:
> lib/setup.c:50:9: error: format not a string literal and no format arguments [-Werror=format-security]
>    50 |         ksft_test_result_skip(buf);
>       |         ^~~~~~~~~~~~~~~~~~~~~
> cc1: some warnings being treated as errors

As the buffer was already pre-printed into, print it as a string
rather than a format-string.

Fixes: cfbab37b ("selftests/net: Add TCP-AO library")
Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Reported-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>