net: skip TestIPv4WriteMsgUDPAddrPort on plan9

This test uses method (*UDPConn).WriteMsgUDPAddrPort, which is
not supported on Plan 9. The test needs to be skipped, like
for example TestAllocs which is already skipped for the
same reason.

Fixes golang#75017

Change-Id: Iaa0e6ecdba0938736d8f675fcac43c46db34cb5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/696095
Auto-Submit: Damien Neil <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: millerresearch

src/net/udpsock_test.go

@@ -710,6 +710,11 @@ func TestIPv6WriteMsgUDPAddrPortTargetAddrIPVersion(t *testing.T) {
 // WriteMsgUDPAddrPort accepts IPv4 and IPv4-mapped IPv6 destination addresses,
 // and rejects IPv6 destination addresses on a "udp4" connection.
 func TestIPv4WriteMsgUDPAddrPortTargetAddrIPVersion(t *testing.T) {
+	switch runtime.GOOS {
+	case "plan9":
+		t.Skipf("not supported on %s", runtime.GOOS)
+	}
+
 	if !testableNetwork("udp4") {
 		t.Skipf("skipping: udp4 not available")
 	}