db/downloader: keep the BT discovery tests off UDP

[AskAlexSharov]

TestBTPeerDiscovery_AddPeersFromENR fails intermittently on the windows-2025 runner:

--- FAIL: TestBTPeerDiscovery_AddPeersFromENR (0.21s)
    bt_peer_discovery_test.go:49: Received unexpected error:
      subsequent listen: listen udp4 :63198: bind: An attempt was made to access
      a socket in a way forbidden by its access permissions.

torrent.NewClient binds the dynamic port the TCP listener was given on UDP as well, for uTP. Windows reserves UDP port ranges, so that second bind can return WSAEACCES; the library retries with fresh ports up to listenAllRetryLimit and then fails NewClient outright.

The tests never needed uTP — peers are injected by TCP address and DHT is already off. Setting DisableUTP (with the NoDHT these configs already set) drops the UDP listeners:

DisableUTP=false → tcp 0.0.0.0:50611, tcp [::]:50611, udp 0.0.0.0:50611, udp [::]:50611
DisableUTP=true  → tcp 0.0.0.0:50614, tcp [::]:50614

The three client configs in the file now go through one helper so a future test cannot reintroduce the UDP bind.

go test ./db/downloader/ -run TestBTPeerDiscovery -count=3 passes.

Not fixed here: the same windows job also failed TestRecordPreload_RecordsElapsedAndBytes (execution/commitment), which is unrelated — a float64 cancellation on a shared counter, not a port problem.