net: Increase the acceptable delay in TestDialerDualstack

This may fix the flakiness on Windows/x64, assuming that it's actually
due to a variance in the connection time which slightly exceeds 100ms.

150ms + 95ms = 245ms, which is still low enough to avoid triggering
Happy Eyeballs (300ms) on non-Windows platforms.

Updates #12309

Change-Id: I816a36fbc0a3e5c90e3cf1b75a134faf0d91557c
Reviewed-on: https://go-review.googlesource.com/14120
Run-TryBot: Paul Marks <pmarks@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dial_test.go

@@ -643,7 +643,7 @@ func TestDialerDualStack(t *testing.T) {
 		}
 	}
 
-	var timeout = 100*time.Millisecond + closedPortDelay
+	var timeout = 150*time.Millisecond + closedPortDelay
 	for _, dualstack := range []bool{false, true} {
 		dss, err := newDualStackServer([]streamListener{
 			{network: "tcp4", address: "127.0.0.1"},