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>