[net]linux: skip if "/proc/sys/net" is not exists on test environment.

net/net_test.go

@@ -3,7 +3,10 @@ package net
 import (
 	"fmt"
 	"os"
+	"runtime"
 	"testing"
+
+	"github.com/shirou/gopsutil/internal/common"
 )
 
 func TestAddrString(t *testing.T) {
@@ -202,6 +205,13 @@ func TestNetFilterCounters(t *testing.T) {
 		return
 	}
 
+	if runtime.GOOS == "linux" {
+		// some test environment has not the path.
+		if !common.PathExists("/proc/sys/net/netfilter/nf_conntrack_count") {
+			t.SkipNow()
+		}
+	}
+
 	v, err := NetFilterCounters()
 	if err != nil {
 		t.Errorf("could not get NetConnections: %v", err)