bpf: Add verifier regression test for previous patch

Add a regression test for var-off zero-sized reads.

Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20231207041150.229139-3-andreimatei1@gmail.com