p2p/discover: add discv5 invalid findnodes result test cases

[vipocenka]

Supersedes #32470.

What

• snap: shorten stall watchdog in eth/protocols/snap/sync_test.go from 1m to 10s.
• discover/v5: consolidate FINDNODE negative tests into a single table-driven test:
  • TestUDPv5_findnodeCall_InvalidNodes covers:
    • invalid IP (unspecified 0.0.0.0) → ignored
    • low UDP port (<=1024) → ignored

Why

• Addresses TODOs:
  • “Make tests smaller” (reduce long 1m timeout).
  • “check invalid IPs”; also cover low port per verifyResponseNode rules (UDP must be >1024).

How it’s validated

• Test-only changes; no production code touched.
• Local runs:
  • go test ./p2p/discover -count=1 -timeout=300s → ok
  • go test ./eth/protocols/snap -count=1 -timeout=600s → ok
• Lint:
  • go run build/ci.go lint → 0 issues on modified files.

Notes

• The test harness uses enode.ValidSchemesForTesting (which includes the “null” scheme), so records signed with enode.SignNull are signature-valid; failures here are due to IP/port validation in verifyResponseNode and netutil.CheckRelayAddr.
• Tests are written as a single table-driven function for clarity; no helpers or environment switching.

[lightclient]

Should be good now. Will wait for CI to go green first, then merge.