nettest: mark test helper functions

Fixes golang/go#30295

Change-Id: Ib98fde1f2f0d166e199238de45ca95aa34164040
Reviewed-on: https://go-review.googlesource.com/c/162925
Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.public.networking@gmail.com>

nettest/conntest.go

@@ -41,6 +41,7 @@ func TestConn(t *testing.T, mp MakePipe) {
 type connTester func(t *testing.T, c1, c2 net.Conn)
 
 func timeoutWrapper(t *testing.T, mp MakePipe, f connTester) {
+	t.Helper()
 	c1, c2, stop, err := mp()
 	if err != nil {
 		t.Fatalf("unable to make pipe: %v", err)
@@ -390,6 +391,7 @@ func testConcurrentMethods(t *testing.T, c1, c2 net.Conn) {
 // checkForTimeoutError checks that the error satisfies the Error interface
 // and that Timeout returns true.
 func checkForTimeoutError(t *testing.T, err error) {
+	t.Helper()
 	if nerr, ok := err.(net.Error); ok {
 		if !nerr.Timeout() {
 			t.Errorf("err.Timeout() = false, want true")
@@ -402,6 +404,7 @@ func checkForTimeoutError(t *testing.T, err error) {
 // testRoundtrip writes something into c and reads it back.
 // It assumes that everything written into c is echoed back to itself.
 func testRoundtrip(t *testing.T, c net.Conn) {
+	t.Helper()
 	if err := c.SetDeadline(neverTimeout); err != nil {
 		t.Errorf("roundtrip SetDeadline error: %v", err)
 	}
@@ -423,6 +426,7 @@ func testRoundtrip(t *testing.T, c net.Conn) {
 // It assumes that everything written into c is echoed back to itself.
 // It assumes that 0xff is not currently on the wire or in the read buffer.
 func resyncConn(t *testing.T, c net.Conn) {
+	t.Helper()
 	c.SetDeadline(neverTimeout)
 	errCh := make(chan error)
 	go func() {