p2p (#4119): treat slow discover v5 tests (#4122)

* configure a 50 ms timeout for tests (like v4 tests)
* use in-memory DB (like v4 tests)
* TestUDPv5_callTimeoutReset: improve speed from 1.2s to 0.2s
* TestUDPv5_callTimeoutReset: reduce the likelihood of "RPC timeout"
* move lookup tests to the "integration" suite
* log details of unmatched packets and sends to non-existing nodes
* fix flaky TestUDPv5_findnodeHandling:

    Table.nextRevalidateTime was random (from 0 to 10s).
    Sometimes it triggered doRevalidate immediately, and it produced an unexpected ping.
    Configure a high interval to not revalidate during the tests.

Time improved from 1.7s to 0.2s.

Test with:

    go test ./p2p/discover -run TestUDPv5 -count 1