Rendering test using virtual screen

[tompng]

This pull request is a PoC of rendering test with virtual screen.

assert_readmultiline(expected: '1.itself', prompt: '>', termination_proc: ->*{true}) do |io|
  io.write '1.ab'
  assert_equal ['>1.ab', ' 1.abs', ' 1.abs2'], io.current_screen
  io.write "\e"
  io.wait 0.49999999999
  assert_equal ['>1.ab', ' 1.abs', ' 1.abs2'], io.current_screen
  io.wait 0.00000000002
  assert_equal ['>1.ab'], io.current_screen
  io.write "\C-h\C-h\C-h.itself"
  io.close
end

Since we introduced differential rendering in #614, LineEditor now have a cache of what LineEditor rendered to the screen. In other words, LineEditor now have a virtual screen internally. We can use it for testing.

We still need yamatanooroti, but we can reduce using it to make CI faster.

Key actor test: use this virtual screen test
Test of render_differential correctly rendered to terminal: use yamatanooroti
Test some edge cases, Test rendering through multiple readmultiline: use yamatanooroti

TODO: specify terminal window size