net: ignore blackhole route in TestDialCancel

CL 496037 had ignored 3 types of null route, however blackhole route
is not included i.e. on Linux we can add a blackhole route by
`ip route add blackhole 198.18.0.254/32`

Fixes #61590

Change-Id: I9ddb86c5be0e5e261afa96cbaf55d9fdf30b2795
Reviewed-on: https://go-review.googlesource.com/c/go/+/513595
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: M Zhuo <mzh@golangcn.org>

src/net/dial_test.go

@@ -784,6 +784,7 @@ func TestDialCancel(t *testing.T) {
 					"connection refused",
 					"unreachable",
 					"no route to host",
+					"invalid argument",
 				}
 				e := err.Error()
 				for _, ignore := range ignorable {