test: do not return spent coins from CCoinsViewTest::GetCoin

Production `GetCoin()` implementations only return unspent coins.

Update the `CCoinsView` test backend to match that contract, so tests stop exercising cache states that cannot occur with `CCoinsViewCache` or `CCoinsViewDB`.

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

src/test/coins_tests.cpp

@@ -48,11 +48,7 @@ public:
 
     std::optional<Coin> GetCoin(const COutPoint& outpoint) const override
     {
-        if (auto it{map_.find(outpoint)}; it != map_.end()) {
+        if (auto it{map_.find(outpoint)}; it != map_.end() && !it->second.IsSpent()) return it->second;
-            if (!it->second.IsSpent() || m_rng.randbool()) {
-                return it->second; // TODO spent coins shouldn't be returned
-            }
-        }
         return std::nullopt;
     }