selftests/bpf: Test unbounded var_off stack access

Test the case when reg->smax_value is too small/big and can overflow,
and separately min and max values outside of stack bounds.

Example of output:
  # ./test_verifier
  #856/p indirect variable-offset stack access, unbounded OK
  #857/p indirect variable-offset stack access, max out of bound OK
  #858/p indirect variable-offset stack access, min out of bound OK

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