selftests/bpf: check if verifier tracks constants spilled by BPF_ST_MEM

Check that verifier tracks the value of 'imm' spilled to stack by
BPF_ST_MEM instruction. Cover the following cases:
- write of non-zero constant to stack;
- write of a zero constant to stack.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230214232030.1502829-3-eddyz87@gmail.com


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