pkg/rpc: move test that needs testplanet

Move rpc test that uses testplanet into private/testplanet.

This ensures that rpc doesn't have the whole system as a dependency
making it easier to separate.

This unfortunately leaves pkg/rpc without specific tests, but
we would need to write new tests that only use the core packages.

Change-Id: I402ab3c2d50282af159c2ef3371d23b0997fef0a

private/testplanet/rpc_test.go

@@ -1,7 +1,7 @@
 // Copyright (C) 2019 Storj Labs, Inc.
 // See LICENSE for copying information.
 
-package rpc_test
+package testplanet_test
 
 import (
 	"context"