selftests/bpf: Test indirect var_off stack access in raw mode

Test that verifier rejects indirect access to uninitialized stack with
variable offset.

Example of output:
  # ./test_verifier
  ...
  #859/p indirect variable-offset stack access, uninitialized OK

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>