ruby: Add test case for referring external package

The new spec uses existing EchoDuplicate service in
grpc.testing.duplicate, which refers grpc.testing.Empty,
and verifies if the plugin generates this correctly as
Grpc::Testing::Empty.

src/ruby/pb/grpc/testing/duplicate/echo_duplicate_services.rb

+# Generated by the protocol buffer compiler.  DO NOT EDIT!
+# Source: src/proto/grpc/testing/duplicate/echo_duplicate.proto for package 'grpc.testing.duplicate'
+
+require 'grpc'
+require 'src/proto/grpc/testing/duplicate/echo_duplicate'
+
+module Grpc
+  module Testing
+    module Duplicate
+      module EchoTestService
+
+        # TODO: add proto service documentation here
+        class Service
+
+          include GRPC::GenericService
+
+          self.marshal_class_method = :encode
+          self.unmarshal_class_method = :decode
+          self.service_name = 'grpc.testing.duplicate.EchoTestService'
+
+          rpc :Echo, Grpc::Testing::EchoRequest, Grpc::Testing::EchoResponse
+        end
+
+        Stub = Service.rpc_stub_class
+      end
+    end
+  end
+end

src/ruby/spec/pb/duplicate/codegen_spec.rb

+# Copyright 2016, Google Inc.
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+#     * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#     * Redistributions in binary form must reproduce the above
+# copyright notice, this list of conditions and the following disclaimer
+# in the documentation and/or other materials provided with the
+# distribution.
+#     * Neither the name of Google Inc. nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+require 'open3'
+require 'tmpdir'
+
+def can_run_codegen_check
+  system('which grpc_ruby_plugin') && system('which protoc')
+end
+
+describe 'Ping protobuf code generation' do
+  if !can_run_codegen_check
+    skip 'protoc || grpc_ruby_plugin missing, cannot verify ping code-gen'
+  else
+    it 'should have the same content as created by code generation' do
+      root_dir = File.join(File.dirname(__FILE__), '..', '..', '..', '..', '..')
+
+      # Get the current content
+      service_path = File.join(root_dir, 'src', 'ruby', 'pb', 'grpc',
+                               'testing', 'duplicate',
+                               'echo_duplicate_services.rb')
+      want = nil
+      File.open(service_path) { |f| want = f.read }
+
+      # Regenerate it
+      plugin, = Open3.capture2('which', 'grpc_ruby_plugin')
+      plugin = plugin.strip
+      got = nil
+      Dir.mktmpdir do |tmp_dir|
+        gen_out = File.join(tmp_dir, 'src', 'proto', 'grpc', 'testing',
+                            'duplicate', 'echo_duplicate_services.rb')
+        pid = spawn(
+          'protoc',
+          '-I.',
+          'src/proto/grpc/testing/duplicate/echo_duplicate.proto',
+          "--grpc_out=#{tmp_dir}",
+          "--plugin=protoc-gen-grpc=#{plugin}",
+          chdir: root_dir)
+        Process.wait(pid)
+        File.open(gen_out) { |f| got = f.read }
+      end
+      expect(got).to eq(want)
+    end
+  end
+end