Golang net: new test TestIPv4WriteMsgUDPAddrPortTargetAddrIPVersion fails on plan9

CL 689976 added a new test which plan9 builders are unable to pass, because function (*UDPConn).writeMsgAddrPort is not supported on Plan 9. Example failure here:

--- FAIL: TestIPv4WriteMsgUDPAddrPortTargetAddrIPVersion (0.01s)
    udpsock_test.go:729: conn.WriteMsgUDPAddrPort(buf, nil, daddr4) failed: write udp 127.0.0.1:62634->127.0.0.1:12345: not supported by plan 9
    udpsock_test.go:733: conn.WriteMsgUDPAddrPort(buf, nil, daddr4in6) failed: write udp 127.0.0.1:62634->[::ffff:127.0.0.1]:12345: not supported by plan 9
FAIL
Running goroutines:
net.(*Dialer).dialCtx.func1()
    /home/swarming/swarming2/.swarming/w/ir/x/w/goroot/src/net/dial.go:577 +0x88
created by net.(*Dialer).dialCtx in goroutine 75
    /home/swarming/swarming2/.swarming/w/ir/x/w/goroot/src/net/dial.go:576 +0x270

FAIL    net 39.467s

Comment From: gopherbot

Change https://go.dev/cl/696095 mentions this issue: net: skip TestIPv4WriteMsgUDPAddrPort on plan9