The test was flaky, because of the "endpoint prediction".
The test starts 5 nodes one by one.
Node 0 is used as a bootstrap node for nodes 1-4.
When it is about to add, say, node 3, nodes 0 and 1 might already have had a chance to communicate,
and updateEndpoints() deletes the node 0 UDP port, because fallbackUDP port was not configured.

In this case node 3 would get a bootstrap node 0 without a port and lead to an error:

    v5_udp_test.go:110: bad bootstrap node "enr:...": missing UDP port

The problem was reproducible by this command:

    go test ./p2p/discover -run TestUDPv5_lookupE2E -count 500