p2p: fix flaky test TestServerPortMapping (ethereum#30241)

The test specifies `ListenAddr: ":0"`, which means a random ephemeral
port will be chosen for the TCP listener by the OS. Additionally, since
no `DiscAddr` was specified, the same port that is chosen automatically
by the OS will also be used for the UDP listener in the discovery UDP
setup. This sometimes leads to test failures if the TCP listener picks a
free TCP port that is already taken for UDP. By specifying `DiscAddr:
":0"`, the UDP port will be chosen independently from the TCP port,
fixing the random failure.

See issue ethereum#29830.

Verified using
```
cd p2p
go test -c -race
stress ./p2p.test -test.run=TestServerPortMapping
...
5m0s: 4556 runs so far, 0 failures
```

The issue described above can technically lead to sporadic failures on
systems that specify a listen address via the `--port` flag of 0 while
not setting `--discovery.port`. Since the default is using port `30303`
and using a random ephemeral port is likely not used much to begin with,
not addressing the root cause might be acceptable.

p2p/server_nat_test.go

@@ -36,6 +36,7 @@ func TestServerPortMapping(t *testing.T) {
 			PrivateKey: newkey(),
 			NoDial:     true,
 			ListenAddr: ":0",
+			DiscAddr:   ":0",
 			NAT:        mockNAT,
 			Logger:     testlog.Logger(t, log.LvlTrace),
 			clock:      clock,