rpctest: Gate rpctest-based behind a build tag.

This adds a new build tag named rpctest which must be set in order for
rpctest-based tests to be executed.  The new build tag is also added to
the goclean.sh script which is executed by Travis during continuous
integration builds.

This change is being made because the rpctest framework requires
additional careful user configuration to ensure the version of btcd
under test can be programmatically launched from the system path with
all of the necessary ports open whereas all of the other tests are
self-contained within the test binary itself.

Since said additional configuration is typically not done, it leads to a
lot of false positives.  Putting the tests behind a build tag allows
them to remain to be available and run during continuous integration
without imposing the additional configuration requirements on users.

goclean.sh

@@ -12,7 +12,7 @@ set -ex
 test -z "$(go fmt $(glide novendor) | tee /dev/stderr)"
 test -z "$(for package in $(glide novendor); do golint $package; done | grep -v 'ALL_CAPS\|OP_\|NewFieldVal' | tee /dev/stderr)"
 test -z "$(go vet $(glide novendor) 2>&1 | tee /dev/stderr)"
-env GORACE="halt_on_error=1" go test -v -race $(glide novendor)
+env GORACE="halt_on_error=1" go test -v -race -tags rpctest $(glide novendor)
 
 # Run test coverage on each subdirectories and merge the coverage profile.
 

rpcserver_test.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
+// This file is ignored during the regular tests due to the following build tag.
+// +build rpctest
+
 package main
 
 import (

rpctest/rpc_harness_test.go

@@ -1,6 +1,10 @@
 // Copyright (c) 2016 The btcsuite developers
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
+
+// This file is ignored during the regular tests due to the following build tag.
+// +build rpctest
+
 package rpctest
 
 import (