net: fix TestDialAddrError

Fixes #15291.

Change-Id: I563140c2acd37d4989a940488b217414cf73f6c2
Reviewed-on: https://go-review.googlesource.com/22077Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/net/error_test.go

@@ -211,6 +211,9 @@ func TestDialAddrError(t *testing.T) {
 	case "nacl", "plan9":
 		t.Skipf("not supported on %s", runtime.GOOS)
 	}
+	if !supportsIPv4 || !supportsIPv6 {
+		t.Skip("both IPv4 and IPv6 are required")
+	}
 
 	for _, tt := range []struct {
 		network string