bpf: selftests: Use C99 initializers in test_sock.c

Use C99 initializers for the initialization of 'tests' in test_sock.c.

Signed-off-by: Menglong Dong <imagedong@tencent.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220106132022.3470772-3-imagedong@tencent.com