p2p: TestUDPv4_Lookup - unsolicited reply error (#4056)

The UDP test must be closed after the serveTestnet exits.
If it happens before, the serveTestnet encounters this error.
(it tries to emulate a packet receival after closing the transport)

FindNode triggers a Ping in ensureBond.
This causes an extra Sleep for "ping back".
Don't wait for this in tests.

Close v5 tests.

The requests may also timeout if a lot of them queue up in the udpTest.pipe,
and serveTestnet is slow to process them.
Increase replyTimeout a bit to prevent that.