Add the rlpx fuzzing thunk test

tests/fuzzing/rlpx/thunk.nim

@@ -0,0 +1,35 @@
+import
+  chronos, eth/p2p, eth/p2p/rlpx, eth/p2p/private/p2p_types,
+  eth/p2p/rlpx_protocols/[whisper_protocol, eth_protocol],
+  ../fuzztest, ../p2p/p2p_test_helper
+
+proc recvMsgMock(msg: openArray[byte]): tuple[msgId: int, msgData: Rlp] =
+  var rlp = rlpFromBytes(@msg.toRange)
+
+  let msgid = rlp.read(int)
+  return (msgId, rlp)
+
+var
+  node1: EthereumNode
+  node2: EthereumNode
+  peer: Peer
+
+# This is not a good example of a fuzzing test and it would be much better
+# to mock more to get rid of anything sockets, async, etc.
+# However, it can and has provided reasonably quick results anyhow.
+init:
+  node1 = setupTestNode(eth, Whisper)
+  node2 = setupTestNode(eth, Whisper)
+
+  node2.startListening()
+  peer = waitFor node1.rlpxConnect(newNode(initENode(node2.keys.pubKey,
+                                                     node2.address)))
+
+test:
+  aflLoop: # This appears to have unstable results with afl-clang-fast, probably
+           # because of undeterministic behaviour due to usage of network/async.
+    try:
+      var (msgId, msgData) = recvMsgMock(payload)
+      waitFor peer.invokeThunk(msgId.int, msgData)
+    except CatchableError as e:
+      debug "Test caused CatchableError", exception=e.name, trace=e.repr, msg=e.msg