selftests/bpf: test if pointer type is tracked for BPF_ST_MEM

Check that verifier tracks pointer types for BPF_ST_MEM instructions
and reports error if pointer types do not match for different
execution branches.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230304011247.566040-3-eddyz87@gmail.com


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