bpf, selftests: Fix warning in snprintf_btf where system() call unchecked

On my systems system() calls are marked with warn_unused_result
apparently. So without error checking we get this warning,

./prog_tests/snprintf_btf.c:30:9: warning: ignoring return value
   of ‘system’, declared with attribute warn_unused_result[-Wunused-result]

Also it seems like a good idea to check the return value anyways
to ensure ping exists even if its seems unlikely.

Fixes: 076a95f5 ("selftests/bpf: Add bpf_snprintf_btf helper tests")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/160141006897.25201.12095049414156293265.stgit@john-Precision-5820-Tower

tools/testing/selftests/bpf/prog_tests/snprintf_btf.c

@@ -27,7 +27,9 @@ void test_snprintf_btf(void)
 		goto cleanup;
 
 	/* generate receive event */
-	(void) system("ping -c 1 127.0.0.1 > /dev/null");
+	err = system("ping -c 1 127.0.0.1 > /dev/null");
+	if (CHECK(err, "system", "ping failed: %d\n", err))
+		goto cleanup;
 
 	if (bss->skip) {
 		printf("%s:SKIP:no __builtin_btf_type_id\n", __func__);