fuzz: improve utxo_snapshot target

Add the possibility of giving more guidance to the creation of the
metadata and/or coins, so that the fuzzer gets the chance
to reach more error conditions in ActivateSnapshot and sometimes
successfully creates a valid snapshot.

This also changes the asserts for the success case that were outdated,
and only didn't result in a crash because the fuzzer wasn't able
to reach this code before.

src/kernel/chainparams.cpp

@@ -495,12 +495,19 @@ public:
         };
 
         m_assumeutxo_data = {
-            {
+            {   // For use by unit tests
                 .height = 110,
                 .hash_serialized = AssumeutxoHash{uint256S("0x6657b736d4fe4db0cbc796789e812d5dba7f5c143764b1b6905612f1830609d1")},
                 .nChainTx = 111,
                 .blockhash = uint256S("0x696e92821f65549c7ee134edceeeeaaa4105647a3c4fd9f298c0aec0ab50425c")
             },
+            {
+                // For use by fuzz target src/test/fuzz/utxo_snapshot.cpp
+                .height = 200,
+                .hash_serialized = AssumeutxoHash{uint256S("0x4f34d431c3e482f6b0d67b64609ece3964dc8d7976d02ac68dd7c9c1421738f2")},
+                .nChainTx = 201,
+                .blockhash = uint256S("0x5e93653318f294fb5aa339d00bbf8cf1c3515488ad99412c37608b139ea63b27"),
+            },
             {
                 // For use by test/functional/feature_assumeutxo.py
                 .height = 299,