Add a couple of test for wallet notifications.
- The original PR dealt with conflicting transactions due to RBF trickery, which is not possible on the eCash blockchain. So the test is different, we simply create conflicting transactions while the nodes are disconnected.
- In the original PR, the test for conflicting transactions does not work correctly. We do not observe the same bug. There is a notification for the conflicting transaction, as expected.
This is a backport of core#18878