ipv6: remove arbitrary deadlines from tests

These deadlines were added back in CL 21080043, apparently in an
attempt to increase code coverage numbers. However, nothing in the
tests actually exercises the deadline logic: the tests would pass even
if these methods were no-ops. Their only apparent effect is to make
the tests flaky on slower builders, and to destroy goroutine traces
if the test should ever happen to deadlock.

Updates golang/go#42064
For golang/go#37319

Change-Id: I530a8f3cb80d6d93d1625bc88f0ec7958d4ec35e
Reviewed-on: https://go-review.googlesource.com/c/net/+/366181
Trust: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

ipv6/multicast_test.go

@@ -10,7 +10,6 @@ import (
 	"os"
 	"runtime"
 	"testing"
-	"time"
 
 	"golang.org/x/net/icmp"
 	"golang.org/x/net/internal/iana"
@@ -100,9 +99,6 @@ func TestPacketConnReadWriteMulticastUDP(t *testing.T) {
 				}
 				t.Fatal(err)
 			}
-			if err := p.SetDeadline(time.Now().Add(200 * time.Millisecond)); err != nil {
-				t.Fatal(err)
-			}
 			cm.HopLimit = i + 1
 			if n, err := p.WriteTo(wb, &cm, &grp); err != nil {
 				t.Fatal(err)
@@ -240,9 +236,6 @@ func TestPacketConnReadWriteMulticastICMP(t *testing.T) {
 				}
 				t.Fatal(err)
 			}
-			if err := p.SetDeadline(time.Now().Add(200 * time.Millisecond)); err != nil {
-				t.Fatal(err)
-			}
 			cm.HopLimit = i + 1
 			if n, err := p.WriteTo(wb, &cm, tt.grp); err != nil {
 				t.Fatal(err)