netmode: use non-zero unittest network

Using zero is a bit dangerous as it's the default type's value, so we can miss
some uninitialized variables when testing.

config/protocol.unit_testnet.yml

@@ -1,5 +1,5 @@
 ProtocolConfiguration:
-  Magic: 0
+  Magic: 42
   SecondsPerBlock: 15
   LowPriorityThreshold: 0.000
   MemPoolSize: 50000

pkg/config/netmode/netmode.go

@@ -10,7 +10,7 @@ const (
 	// PrivNet contains magic code usually used for NEO private networks.
 	PrivNet Magic = 56753 // docker privnet
 	// UnitTestNet is a stub magic code used for testing purposes.
-	UnitTestNet Magic = 0
+	UnitTestNet Magic = 42
 )
 
 // Magic describes the network the blockchain will operate on.