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>