selftests/bpf: Fix unused-result warning in snprintf_btf.c

selftests/bpf: Fix unused-result warning in snprintf_btf.c

Daniel reports:

+    system("ping -c 1 127.0.0.1 > /dev/null");

This generates the following new warning when compiling BPF selftests:

  [...]
  EXT-OBJ  [test_progs] cgroup_helpers.o
  EXT-OBJ  [test_progs] trace_helpers.o
  EXT-OBJ  [test_progs] network_helpers.o
  EXT-OBJ  [test_progs] testing_helpers.o
  TEST-OBJ [test_progs] snprintf_btf.test.o
/root/bpf-next/tools/testing/selftests/bpf/prog_tests/snprintf_btf.c: In function ‘test_snprintf_btf’:
/root/bpf-next/tools/testing/selftests/bpf/prog_tests/snprintf_btf.c:30:2: warning: ignoring return value of ‘system’, declared with attribute warn_unused_result [-Wunused-result]
  system("ping -c 1 127.0.0.1 > /dev/null");
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  [...]

Fixes: 076a95f5aff2 ("selftests/bpf: Add bpf_snprintf_btf helper tests")
Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/1601379151-21449-2-git-send-email-alan.maguire@oracle.com