remove UTF-8 strings from mruby-string-ext test

mrbgems/mruby-string-ext/test/string.rb

@@ -167,8 +167,8 @@ assert('String#chr') do
 end
 
 assert('String#lines') do
-  assert_equal ["こん\n", "にちは\n", "世界!"], "こん\nにちは\n世界!".lines
-  assert_equal ["こん\n", "にちは\n", "世界!\n"], "こん\nにちは\n世界!\n".lines
+  assert_equal ["Hel\n", "lo\n", "World!"], "Hel\nlo\nWorld!".lines
+  assert_equal ["Hel\n", "lo\n", "World!\n"], "Hel\nlo\nWorld!\n".lines
   assert_equal ["\n", "\n", "\n"], "\n\n\n".lines
   assert_equal [], "".lines
 end