Add test_not_drain_on_shutdown

test/test_puma_server.rb

@@ -1190,17 +1190,29 @@ def test_custom_io_selector
     assert_equal selector.backend, backend
   end
 
-  def test_drain_on_shutdown
+  def test_drain_on_shutdown(drain=true)
+    num_connections = 10
+
     wait = Queue.new
-    server_run(drain_on_shutdown: true, max_threads: 1) do
+    server_run(drain_on_shutdown: drain, max_threads: 1) do
       wait.pop
       [200, {}, ["DONE"]]
     end
-    connections = Array.new(10) {send_http "GET / HTTP/1.0\r\n\r\n"}
+    connections = Array.new(num_connections) {send_http "GET / HTTP/1.0\r\n\r\n"}
     @server.stop
     wait.close
+    bad = 0
     connections.each do |s|
-      assert_match 'DONE', s.read
+      begin
+        assert_match 'DONE', s.read
+      rescue Errno::ECONNRESET
+        bad += 1
+      end
     end
+    assert_equal drain ? 0 : num_connections - 2, bad
+  end
+
+  def test_not_drain_on_shutdown
+    test_drain_on_shutdown false
   end
 end