Update the Fixnum#chr test for UTF-8.

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

@@ -5,6 +5,11 @@ assert('Integer#chr') do
   assert_equal("A", 65.chr)
   assert_equal("B", 0x42.chr)
 
-  # multibyte encoding (not support yet)
-  assert_raise(RangeError) { 12345.chr }
+  if 255.chr.bytes == [195, 191] then
+      # UTF-8 gem is configured
+      assert_raise(RangeError) { 0x110000.chr }
+  else
+      # multibyte encoding (not support yet)
+      assert_raise(RangeError) { 256.chr }
+  end
 end