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.

ninja all check-all
./contrib/teamcity/build-configurations.py build-ubsan