selftests/bpf: fix tests due to const spill/fill

fix tests that incorrectly assumed that the verifier
cannot track constants through stack.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>