- Refactor _run_mpp so that it takes only one set of arguments.
Success and failure conditions are now tested by calling
_run_mpp multiple times.
- In test_payment_multipart with_timeout, check that the received
failure message actually is MPP_TIMEOUT, and not a generic
failure. Since the onion is obfuscated by the forwarding node,
this tests that obfuscate_onion_packet and decode_onion_packet
work as expected.