p2p/discover: fix TestUDPv4_findnode data race with revalidation ping

[hero5512]

The test was intermittently failing because PING packets from background node revalidation sometimes arrived before the expected NEIGHBORS response. The failure can be seen in https://github.com/ethereum/go-ethereum/actions/runs/18759496644/job/53520537257.

--- FAIL: TestUDPv4_findnode (0.02s)
v4_udp.go:532: TRACE[10-23|19:33:31.361] >> PING/v4 id=e91c8fbf09909f1e addr=10.13.0.1:2000 err=
v4_udp.go:583: TRACE[10-23|19:33:31.368] << FINDNODE/v4 id=2ba2870c5c54644f addr=10.0.1.99:30303 err=
v4_udp.go:532: TRACE[10-23|19:33:31.368] >> NEIGHBORS/v4 id=2ba2870c5c54644f addr=10.0.1.99:30303 err=
v4_udp_test.go:285: sent packet type mismatch, got: *v4wire.Ping, want: *v4wire.Neighbors
FAIL
FAIL    github.com/ethereum/go-ethereum/p2p/discover   15.589s

[fjl]

You can simplify the implementation by using test.waitPacketOut(func(p v4wire.Packet, ... and performing a switch inside of the function. This is preferable to inlining waitPacketOut.

[hero5512]

You can simplify the implementation by using test.waitPacketOut(func(p v4wire.Packet, ... and performing a switch inside of the function. This is preferable to inlining waitPacketOut.

done