Add tests for responding to inbound channel reqs

Add tests for responding to inbound channel reqs

Add functional tests for manually responding to inbound channel requests.
Responding to inbound channel requests are required when the
`manually_accept_inbound_channels` config flag is set to true.

The tests cover the following cases:
* Accepting an inbound channel request
* Rejecting an inbound channel request
* FundingCreated message sent by the counterparty before accepting the
inbound channel request
* Attempting to accept an inbound channel request twice
* Attempting to accept an unkown inbound channel