Wait for each auto-indent update (#918)

Synchronize each input line with its rendered prompt to avoid flaky
auto-indent assertions.

Author: nobu

test/reline/yamatanooroti/test_rendering.rb

@@ -758,6 +758,7 @@ def test_auto_indent
       start_terminal(10, 30, %W{ruby -I#{@pwd}/lib #{@pwd}/test/reline/yamatanooroti/multiline_repl --auto-indent}, startup_message: 'Multiline REPL.')
       "def hoge\nputs(\n1,\n2\n)\nend".lines do |line|
         write line
+        assert_screen(/#{Regexp.escape(line.chomp)}\nprompt>/) if line.end_with?("\n")
       end
       assert_screen(<<~EOC)
         Multiline REPL.