Simplify return values of GetCoin/HaveCoin(InCache)

This removes the possibility for GetCoin/HaveCoin/HaveCoinInCache to return
true while the respective coin is spent. By doing it across all calls, some
extra checks can be eliminated.

coins_tests is modified to call HaveCoin sometimes before and sometimes
after AccessCoin. A further change is needed because the semantics for
GetCoin slightly changed, causing a pruned entry in the parent cache to not
be pulled into the child in FetchCoin.

src/txmempool.cpp

@@ -903,11 +903,7 @@ bool CCoinsViewMemPool::GetCoin(const COutPoint &outpoint, Coin &coin) const {
             return false;
         }
     }
-    return (base->GetCoin(outpoint, coin) && !coin.IsSpent());
-}
-
-bool CCoinsViewMemPool::HaveCoin(const COutPoint &outpoint) const {
-    return mempool.exists(outpoint) || base->HaveCoin(outpoint);
+    return base->GetCoin(outpoint, coin);
 }
 
 size_t CTxMemPool::DynamicMemoryUsage() const {