lnd: add infrastructure for integration testing
lnd: add infrastructure for integration testing

This commit introduces a simple piece of infrastructure for integration
testing: the networkHarness.

The harness allows for the creation and orchestration of a small
testbed of lnd nodes. The harness includes an instance of btcd’s
rpctest.Harness which allows driving certain actions in a mock Bitcoin
network to exercise relevant lnd behavior(s). By default, the harness
creates two instances of lnd (Alice and Bob), then establishes a
peer-to-peer connection between them. Created lnd nodes within the test
network can be driven programmatically by gRPC connections to their
respective RPC severs.

Later on more features will be added such as adding additional nodes,
opening channels between nodes, creating scripting topologies, etc,

AndrewSamokhvalov/lnd,ddustin/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd,bryanvu/lnd,bryanvu/lnd,ddustin/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd