Improve (pre-flight) probing test coverage #2585
Description
In #2534 we introduced pre-flight probing and a corresponding test case.
However, as noted in #2534 (comment), the test case currently relies on checking the logs, which can be improved upon.
As a follow-up, we should DRY up the MPP-probing test logic, add more tests on this basis. In particular we should test the following things independently: a) all MPP probes are executed b) some are excecuted, some are avoided due to liquidity constraints c) we avoid probing for possibly announced last hops.