skip the error log in TestAllocatePortInUse

We know we're going to see an error, so don't log it too:

	$ go test -run TestAllocatePortInUse
	2019/02/21 17:11:34 ERROR: pool could not allocate runner ...
	PASS
	ok      github.com/chromedp/chromedp    0.004s

pool_test.go

@@ -24,7 +24,11 @@ func TestAllocatePortInUse(t *testing.T) {
 	// make the pool use the port already in use via a port range
 	_, portStr, _ := net.SplitHostPort(l.Addr().String())
 	port, _ := strconv.Atoi(portStr)
-	pool, err := NewPool(PortRange(port, port+1))
+	pool, err := NewPool(
+		PortRange(port, port+1),
+		// skip the error log from the used port
+		PoolLog(nil, nil, func(string, ...interface{}) {}),
+	)
 	if err != nil {
 		t.Fatal(err)
 	}