Mark skipUnlessRoot and minKernelRequired as test helpers

With this, the skip message will display the position of the
helper invocation (i.e. the test that caused the skip) instead of the
position of the t.Skip call in minKernelRequired the helper.

netlink_test.go

@@ -24,6 +24,8 @@ import (
 type tearDownNetlinkTest func()
 
 func skipUnlessRoot(t *testing.T) {
+	t.Helper()
+
 	if os.Getuid() != 0 {
 		t.Skip("Test requires root privileges.")
 	}
@@ -187,6 +189,8 @@ func remountSysfs() error {
 }
 
 func minKernelRequired(t *testing.T, kernel, major int) {
+	t.Helper()
+
 	k, m, err := KernelVersion()
 	if err != nil {
 		t.Fatal(err)