[ruby/reline] Add completion_append_character test

(https://github.com/ruby/reline/pull/773)

https://github.com/ruby/reline/commit/5f5a0aa78c

test/reline/test_key_actor_emacs.rb

@@ -939,6 +939,22 @@ class Reline::KeyActor::EmacsTest < Reline::TestCase
     assert_line_around_cursor('foo', '')
   end
 
+  def test_completion_append_character
+    @line_editor.completion_proc = proc { |word|
+      %w[foo_ foo_foo foo_bar].select { |s| s.start_with? word }
+    }
+    @line_editor.completion_append_character = 'X'
+    input_keys('f')
+    input_keys("\C-i", false)
+    assert_line_around_cursor('foo_', '')
+    input_keys('f')
+    input_keys("\C-i", false)
+    assert_line_around_cursor('foo_fooX', '')
+    input_keys(' foo_bar')
+    input_keys("\C-i", false)
+    assert_line_around_cursor('foo_fooX foo_barX', '')
+  end
+
   def test_completion_with_completion_ignore_case
     @line_editor.completion_proc = proc { |word|
       %w{