selftests/bpf: Add unit tests for new sdiv/smod insns

Add unit tests for sdiv/smod insns.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230728011321.3720500-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>