test_yamatanooroti: avoid a forced line break behavior

Windows console that does not support VT sequence 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 affects to a test result.

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.
         >
         オー
         グ言▄