Add checkBlock to Mining interface

Use it in miner_tests.

The getblocktemplate and generateblock RPC calls don't use this,
because it would make the code more verbose.

src/node/interfaces.cpp

@@ -984,6 +984,15 @@ public:
         return std::make_unique<BlockTemplateImpl>(assemble_options, BlockAssembler{chainman().ActiveChainstate(), context()->mempool.get(), assemble_options}.CreateNewBlock(), m_node);
     }
 
+    bool checkBlock(const CBlock& block, const node::BlockCheckOptions& options, std::string& reason, std::string& debug) override
+    {
+        LOCK(chainman().GetMutex());
+        BlockValidationState state{TestBlockValidity(chainman().ActiveChainstate(), block, /*check_pow=*/options.check_pow, /*=check_merkle_root=*/options.check_merkle_root)};
+        reason = state.GetRejectReason();
+        debug = state.GetDebugMessage();
+        return state.IsValid();
+    }
+
     NodeContext* context() override { return &m_node; }
     ChainstateManager& chainman() { return *Assert(m_node.chainman); }
     KernelNotifications& notifications() { return *Assert(m_node.notifications); }