selftests/bpf: Add a few tests to cover

Add three unit tests in verifier_movsx.c to cover
cases where missed var_off setting can cause
unexpected verification success or failure.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240615174637.3995589-1-yonghong.song@linux.dev


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