Use `__ENCODING__` in tests
It cannot be used for `String#size` test if judging whether or not `MRB_UTF8_STRING` is defined by result of `String#size`.
MODIFY
mrbgems/mruby-string-ext/test/string.rb
mrbgems/mruby-string-ext/test/string.rb
##
# String(Ext) Test
-UTF8STRING = ("\343\201\202".size == 1)
+UTF8STRING = __ENCODING__ == "UTF-8"
assert('String#getbyte') do
str1 = "hello"

MODIFY
test/t/string.rb
test/t/string.rb
##
# String ISO Test
-UTF8STRING = ("\343\201\202".size == 1)
+UTF8STRING = __ENCODING__ == "UTF-8"
assert('String', '15.2.10') do
assert_equal Class, String.class

Use `__ENCODING__` in tests

It cannot be used for `String#size` test if judging whether or not `MRB_UTF8_STRING` is defined by result of `String#size`.