Netpoll must be always be initialized when TestNetpollBreak is launched.
However, when it is run in standalone, it won't be the case, so it must
be forced.
Updates: #27707
Change-Id: I28147f3834f3d6aca982c6a298feadc09b55f66e
Reviewed-on: https://go-review.googlesource.com/c/go/+/204058
Run-TryBot: Clément Chigot <clement.chigot%atos.net@gtempaccount.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>