runtime: initialize netpoll in TestNetpollBreak

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>

src/runtime/export_test.go

@@ -40,6 +40,8 @@ var Usleep = usleep
 
 var PhysHugePageSize = physHugePageSize
 
+var NetpollGenericInit = netpollGenericInit
+
 type LFNode struct {
 	Next    uint64
 	Pushcnt uintptr

src/runtime/proc_test.go

@@ -992,7 +992,7 @@ func TestNetpollBreak(t *testing.T) {
 	}
 
 	// Make sure that netpoll is initialized.
-	time.Sleep(1)
+	runtime.NetpollGenericInit()
 
 	start := time.Now()
 	c := make(chan bool, 2)