rpctest: Gate rpctest-based behind a build tag.

Contains upstream commits:
- 99165eb558
  - This is a NOOP since it was already done in Decred when porting the
    rpctest framework
- 754c4fbe0c

The merge commit also ports the necessary changes to run the tests in
the new Decred build setup.

rpcserver_test.go

@@ -3,6 +3,9 @@
 // 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

@@ -2,6 +2,10 @@
 // Copyright (c) 2017 The Decred 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 (

run_tests.sh

@@ -26,7 +26,9 @@ TESTCMD="test -z \"\$(gometalinter --disable-all \
   --enable=unconvert \
   --vendor \
   --deadline=10m . | tee /dev/stderr)\"&& \
-  env GORACE='halt_on_error=1' go test -short -race \$(glide novendor)"
+  env GORACE='halt_on_error=1' go test -short -race \
+  -tags rpctest \
+  \$(glide novendor)"
 
 if [ $GOVERSION == "local" ]; then
     eval $TESTCMD