selftests/bpf: Add selftest for allow_ptr_leaks

selftests/bpf: Add selftest for allow_ptr_leaks

- Without prev commit

  $ tools/testing/selftests/bpf/test_progs --name=tc_bpf
  #232/1   tc_bpf/tc_bpf_root:OK
  test_tc_bpf_non_root:PASS:set_cap_bpf_cap_net_admin 0 nsec
  test_tc_bpf_non_root:PASS:disable_cap_sys_admin 0 nsec
  0: R1=ctx(off=0,imm=0) R10=fp0
  ; if ((long)(iph + 1) > (long)skb->data_end)
  0: (61) r2 = *(u32 *)(r1 +80)         ; R1=ctx(off=0,imm=0) R2_w=pkt_end(off=0,imm=0)
  ; struct iphdr *iph = (void *)(long)skb->data + sizeof(struct ethhdr);
  1: (61) r1 = *(u32 *)(r1 +76)         ; R1_w=pkt(off=0,r=0,imm=0)
  ; if ((long)(iph + 1) > (long)skb->data_end)
  2: (07) r1 += 34                      ; R1_w=pkt(off=34,r=0,imm=0)
  3: (b4) w0 = 1                        ; R0_w=1
  4: (2d) if r1 > r2 goto pc+1
  R2 pointer comparison prohibited
  processed 5 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
  test_tc_bpf_non_root:FAIL:test_tc_bpf__open_and_load unexpected error: -13
  #233/2   tc_bpf_non_root:FAIL

- With prev commit

  $ tools/testing/selftests/bpf/test_progs --name=tc_bpf
  #232/1   tc_bpf/tc_bpf_root:OK
  #232/2   tc_bpf/tc_bpf_non_root:OK
  #232     tc_bpf:OK
  Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230823020703.3790-3-laoar.shao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>