net: deflake TestDialTimeoutMaxDuration

Fixes #15745.

Change-Id: I6f9a1dcf0b1d97cb443900c7d8da09ead83d4b6a
Reviewed-on: https://go-review.googlesource.com/23243
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/timeout_test.go

@@ -124,7 +124,7 @@ func TestDialTimeoutMaxDuration(t *testing.T) {
 
 	for i, tt := range dialTimeoutMaxDurationTests {
 		ch := make(chan error)
-		max := time.NewTimer(100 * time.Millisecond)
+		max := time.NewTimer(250 * time.Millisecond)
 		defer max.Stop()
 		go func() {
 			d := Dialer{Timeout: tt.timeout}