Add a test for Readline.readline without tty

test/readline/test_readline.rb

@@ -3,6 +3,7 @@
 require "test/unit"
 require "tempfile"
 require "timeout"
+require "open3"
 
 module BasetestReadline
   INPUTRC = "INPUTRC"
@@ -809,6 +810,19 @@ def test_completion_quote_character_after_completion
     Readline.completer_quote_characters = saved_completer_quote_characters if saved_completer_quote_characters
   end
 
+  def test_without_tty
+    loader = nil
+    if defined?(TestReadline) && self.class == TestReadline
+      loader = "use_ext_readline"
+    elsif defined?(TestRelineAsReadline) && self.class == TestRelineAsReadline
+      loader = "use_lib_reline"
+    end
+    if loader
+      res, exit_status = Open3.capture2e("ruby -I#{__dir__} -Ilib -rhelper -e '#{loader}; Readline.readline(%{y or n?})'", stdin_data: "y\n")
+      assert exit_status.success?, "It should work fine without tty, but it failed.\nError output:\n#{res}"
+    end
+  end
+
   private
 
   def replace_stdio(stdin_path, stdout_path)