Add whisper encode decode test and rename rlp test

tests/fuzzing/whisper/encode_decode.nim

@@ -0,0 +1,18 @@
+import
+  options, sequtils, chronicles,
+  eth/p2p/rlpx_protocols/whisper_protocol as whisper,
+  ../fuzztest
+
+test:
+  let
+    data = @payload.distribute(2)
+    whisperPayload = Payload(payload: data[0], padding: some(data[1]))
+    encoded = whisper.encode(whisperPayload)
+
+    decoded = whisper.decode(encoded.get())
+
+  doAssert data[0] == decoded.get().payload
+  if data[1].len > 0:
+    doAssert data[1] == decoded.get().padding.get()
+  else:
+    doAssert decoded.get().padding.isNone()