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/interfaces/mining.h

@@ -114,6 +114,21 @@ public:
      */
     virtual std::unique_ptr<BlockTemplate> createNewBlock(const node::BlockCreateOptions& options = {}) = 0;
 
+    /**
+     * Checks if a given block is valid.
+     *
+     * @param[in] block       the block to check
+     * @param[in] options     verification options: the proof-of-work check can be
+     *                        skipped in order to verify a template generated by
+     *                        external software.
+     * @param[out] reason     failure reason (BIP22)
+     * @param[out] debug      more detailed rejection reason
+     * @returns               whether the block is valid
+     *
+     * For signets the challenge verification is skipped when check_pow is false.
+     */
+    virtual bool checkBlock(const CBlock& block, const node::BlockCheckOptions& options, std::string& reason, std::string& debug) = 0;
+
     //! Get internal node context. Useful for RPC and testing,
     //! but not accessible across processes.
     virtual node::NodeContext* context() { return nullptr; }