add nonce to mock transaction for easier debugging

tests/mysticeti/mocks/transacting.nim

@@ -1,8 +1,10 @@
+import std/random
 import mysticeti/dependencies/transacting
 
 type
   MockTransaction = object
+    nonce: int
   MockTransacting* = Transacting[MockTransaction]
 
 proc example*(_: type MockTransaction): MockTransaction =
-  discard
+  MockTransaction(nonce: rand(int))

tests/mysticeti/validator/testValidatorNetwork.nim

@@ -306,7 +306,7 @@ suite "Validator Network":
       author = CommitteeMember(0),
       round = 0,
       parents = @[],
-      transactions = seq[Transaction].example
+      transactions = seq[Transaction].example(length = 1..10)
     ))
     # validator 0 sends different proposals to different parts of the network
     !exchangeBlock(simulator.validators[0], simulator.validators[0], proposalA)