lntest/itest: expand testSingleHopSendToRoute matrix

In this commit, we refactor the testSingleHopSendToRoute test to support
table driven tests for various endpoints and payment types. Currently
only the main rpcserver's SendToRoute is tested, so we also add
support the SendToRouteSync and the routerrpc's SendToRoute.

The tests are also modified to have each endpoint perform a single-hop,
single-shot MPP payment. This asserts that the Hop messages are being
properly unmarshalled and that setting correctly yields a successful
payment. At the momemnt the receiver does not actually verify or use the
MPP fields presented in the onion, though this test will be expanded
later as those pieces are assembled.