Add core assertions for assert_warn

Gemfile

@@ -6,4 +6,5 @@ group :development do
   gem "bundler"
   gem "rake"
   gem "test-unit"
+  gem "test-unit-ruby-core", git: "https://github.com/ruby/test-unit-ruby-core"
 end

test/net/smtp/test_smtp.rb

@@ -2,6 +2,9 @@
 
 require 'net/smtp'
 require 'test/unit'
+require "core_assertions"
+
+Test::Unit::TestCase.include Test::Unit::CoreAssertions
 
 module Net
   class TestSMTP < Test::Unit::TestCase
@@ -628,6 +631,19 @@ def test_quit!
       smtp.quit!
     end
 
+    def test_quit_and_warn
+      server = FakeServer.start
+      def server.quit
+        @sock.puts "400 BUSY\r\n"
+      end
+      smtp = Net::SMTP.start 'localhost', server.port
+      assert_warn(/SMTPServerBusy during .*#quit!/i) do
+        smtp.quit!
+      end
+      assert_equal "QUIT\r\n", server.commands.last
+      refute smtp.started?
+    end
+
     def test_quit_and_reraise
       server = FakeServer.start
       def server.quit