test_yamatanooroti: avoid windows conhost behavior

Non VT sequence supporting windows console has forced newline output when writing rightmost column of line. This may appear as an additional line break.
Clearing screen to avoid this environment dependent behavior.

Author: YO4

test/reline/yamatanooroti/test_rendering.rb

@@ -1256,11 +1256,9 @@ def test_dialog_with_fullwidth_chars
         start_terminal(20, 5, %W{ruby -I#{@pwd}/lib #{@pwd}/test/reline/yamatanooroti/multiline_repl --dialog fullwidth,scrollkey,scrollbar}, startup_message: 'Multiline REPL.', codepage: 932)
         omit "codepage 932 not supported" if !codepage_success?
       end
+      write("\C-l")
       6.times{ write('j') }
       assert_screen(<<~'EOC')
-        Multi
-        line
-        REPL.
         >
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