selftests/bpf: Add selftest for PTR_UNTRUSTED

Add a new selftest to check the PTR_UNTRUSTED condition. Below is the
result,

 #160     ptr_untrusted:OK

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230713025642.27477-5-laoar.shao@gmail.com


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