runtime: disable preemption test (fix build)

TBR=golang-dev
CC=golang-dev
https://golang.org/cl/10849043

src/pkg/runtime/proc_test.go

@@ -169,6 +169,7 @@ var preempt = func() int {
 }
 
 func TestPreemptionGC(t *testing.T) {
+	t.Skip("preemption is disabled")
 	// Test that pending GC preempts running goroutines.
 	const P = 5
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(P + 1))