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.