This commit introduces a new file: rpcserver_test.go dedicated for
including integration tests for btcd using the new rpctest package.

The tests are created using a TestMain instance first creates a single
main harness which is intended to be re-used across tests instances.
Afterwards all registered RPC tests are executed, with proper clean up
being executed regardless of the passing state of the tests.

The following RPC calls are excessed by the initial set of tests added:
    * getbestblock
    * getblockcount
    * getblockhash

Upstream commit 2d86fbf