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.
Backport of core PR19098.
Depends on D7992.