TestDialParallel, TestDialerFallbackDelay and TestDialCancel
require dialTCP to support cancellation, which has been
implemented for Plan 9 in CL 22144.

Updates #11225.
Updates #11932.

Change-Id: I3b30a645ef79227dfa519cde8d46c67b72f2485c
Reviewed-on: https://go-review.googlesource.com/22203
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>