test: Remove duplicate NodeContext hacks

Qt tests currently are currently using two NodeContext structs at the same
time, one in interfaces::NodeImpl::m_context, and the other in
BasicTestingSetup::m_node, and the tests have hacks transferring state between
them.

Fix this by getting rid of the NodeImpl::m_context struct and making it a
pointer. This way a common BitcoinApplication object can be used for all qt
tests, but they can still have their own testing setups.

Non-test code is changing but non-test behavior is still the same as before.

Motivation for this PR is to be able to remove the
"std::move(test.m_node.connman)" and mempool hacks for swapping individual
NodeContext members in Qt tests, because followup PR #19099 adds yet another
member (wallet_client) that needs to be swapped. After this change, the whole
NodeContext struct can be swapped instead of individual members, so the
workarounds are less fragile and invasive.

src/interfaces/node.h

@@ -263,12 +263,14 @@ public:
         std::function<void(SynchronizationState, interfaces::BlockTip tip, double verification_progress)>;
     virtual std::unique_ptr<Handler> handleNotifyHeaderTip(NotifyHeaderTipFn fn) = 0;
 
-    //! Return pointer to internal chain interface, useful for testing.
+    //! Get and set internal node context. Useful for testing, but not
+    //! accessible across processes.
     virtual NodeContext* context() { return nullptr; }
+    virtual void setContext(NodeContext* context) { }
 };
 
 //! Return implementation of Node interface.
-std::unique_ptr<Node> MakeNode();
+std::unique_ptr<Node> MakeNode(NodeContext* context = nullptr);
 
 //! Block tip (could be a header or not, depends on the subscribed signal).
 struct BlockTip {