selftests/bpf: Add testing of log_buf==NULL condition for BPF_PROG_LOAD

Add few extra test conditions to validate that it's ok to pass
log_buf==NULL and log_size==0 to BPF_PROG_LOAD command with the intent
to get log_true_size without providing a buffer.

Test that log_buf==NULL condition *does not* return -ENOSPC.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/bpf/20230406234205.323208-19-andrii@kernel.org