selftests/bpf: Add unit tests for new sign-extension mov insns

Add unit tests for movsx insns.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230728011309.3719295-1-yonghong.song@linux.dev


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