selftests/bpf: Add test for sign extension in coerce_subreg_to_size_sx()

selftests/bpf: Add test for sign extension in coerce_subreg_to_size_sx()

Add a test for unsigned ranges after signed extension instruction. This
case isn't currently covered by existing tests in verifier_movsx.c.

Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Dimitar Kanaliev <dimitar.kanaliev@siteground.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20241014121155.92887-4-dimitar.kanaliev@siteground.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>