fuzz: move backend mutating block to end of coins_view

Refactor TestCoinsView() to move code that directly modifies
backend_coins_view to the end of the function.
This prepares for a CoinsViewOverlay fuzz target that asserts
the backend_coins_view is not mutated by any methods before
BatchWrite is called.

Co-authored-by: l0rinc <pap.lorinc@gmail.com>

src/test/fuzz/coins_view.cpp

@@ -177,31 +177,6 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsViewCache& co
             });
     }
 
-    {
-        const Coin& coin_using_access_coin = coins_view_cache.AccessCoin(random_out_point);
-        const bool exists_using_access_coin = !(coin_using_access_coin == EMPTY_COIN);
-        const bool exists_using_have_coin = coins_view_cache.HaveCoin(random_out_point);
-        const bool exists_using_have_coin_in_cache = coins_view_cache.HaveCoinInCache(random_out_point);
-        if (auto coin{coins_view_cache.GetCoin(random_out_point)}) {
-            assert(*coin == coin_using_access_coin);
-            assert(exists_using_access_coin && exists_using_have_coin_in_cache && exists_using_have_coin);
-        } else {
-            assert(!exists_using_access_coin && !exists_using_have_coin_in_cache && !exists_using_have_coin);
-        }
-        // If HaveCoin on the backend is true, it must also be on the cache if the coin wasn't spent.
-        const bool exists_using_have_coin_in_backend = backend_coins_view.HaveCoin(random_out_point);
-        if (!coin_using_access_coin.IsSpent() && exists_using_have_coin_in_backend) {
-            assert(exists_using_have_coin);
-        }
-        if (auto coin{backend_coins_view.GetCoin(random_out_point)}) {
-            assert(exists_using_have_coin_in_backend);
-            // Note we can't assert that `coin_using_get_coin == *coin` because the coin in
-            // the cache may have been modified but not yet flushed.
-        } else {
-            assert(!exists_using_have_coin_in_backend);
-        }
-    }
-
     {
         bool expected_code_path = false;
         try {
@@ -305,6 +280,31 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsViewCache& co
                 (void)IsWitnessStandard(CTransaction{random_mutable_transaction}, coins_view_cache);
             });
     }
+
+    {
+        const Coin& coin_using_access_coin = coins_view_cache.AccessCoin(random_out_point);
+        const bool exists_using_access_coin = !(coin_using_access_coin == EMPTY_COIN);
+        const bool exists_using_have_coin = coins_view_cache.HaveCoin(random_out_point);
+        const bool exists_using_have_coin_in_cache = coins_view_cache.HaveCoinInCache(random_out_point);
+        if (auto coin{coins_view_cache.GetCoin(random_out_point)}) {
+            assert(*coin == coin_using_access_coin);
+            assert(exists_using_access_coin && exists_using_have_coin_in_cache && exists_using_have_coin);
+        } else {
+            assert(!exists_using_access_coin && !exists_using_have_coin_in_cache && !exists_using_have_coin);
+        }
+        // If HaveCoin on the backend is true, it must also be on the cache if the coin wasn't spent.
+        const bool exists_using_have_coin_in_backend = backend_coins_view.HaveCoin(random_out_point);
+        if (!coin_using_access_coin.IsSpent() && exists_using_have_coin_in_backend) {
+            assert(exists_using_have_coin);
+        }
+        if (auto coin{backend_coins_view.GetCoin(random_out_point)}) {
+            assert(exists_using_have_coin_in_backend);
+            // Note we can't assert that `coin_using_get_coin == *coin` because the coin in
+            // the cache may have been modified but not yet flushed.
+        } else {
+            assert(!exists_using_have_coin_in_backend);
+        }
+    }
 }
 
 FUZZ_TARGET(coins_view, .init = initialize_coins_view)