net: slowDialTCP should wait forever if no deadline exists.

This allows TestDialerFallbackDelay to pass again on machines where IPv6
connections to nowhere fail quickly instead of hanging.

This bug appeared last month, when I deleted the slowTimeout constant.

Updates #11225
Fixes #14731

Change-Id: I840011eee571aab1041022411541736111c7fad5
Reviewed-on: https://go-review.googlesource.com/20493
Run-TryBot: Paul Marks <pmarks@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/net/dial_test.go

@@ -238,9 +238,14 @@ const (
 func slowDialTCP(net string, laddr, raddr *TCPAddr, deadline time.Time, cancel <-chan struct{}) (*TCPConn, error) {
 	c, err := dialTCP(net, laddr, raddr, deadline, cancel)
 	if ParseIP(slowDst4).Equal(raddr.IP) || ParseIP(slowDst6).Equal(raddr.IP) {
+		// Wait for the deadline, or indefinitely if none exists.
+		var wait <-chan time.Time
+		if !deadline.IsZero() {
+			wait = time.After(deadline.Sub(time.Now()))
+		}
 		select {
 		case <-cancel:
-		case <-time.After(deadline.Sub(time.Now())):
+		case <-wait:
 		}
 	}
 	return c, err