ipc mining: Prevent `Assertion m_node.chainman' failed`` errors on early startup

This fixes ``Assertion `m_node.chainman' failed`` errors first reported
https://github.com/bitcoin/bitcoin/issues/33994#issuecomment-3602551596 when
IPC mining methods are called before ChainstateManager is loaded.

The fix works by making the `Init.makeMining` method block until chainstate
data is loaded.

src/test/miner_tests.cpp

@@ -67,7 +67,7 @@ struct MinerTestingSetup : public TestingSetup {
     }
     std::unique_ptr<Mining> MakeMining()
     {
-        return interfaces::MakeMining(m_node);
+        return interfaces::MakeMining(m_node, /*wait_loaded=*/false);
     }
 };
 } // namespace miner_tests