* test/ruby/envutil.rb (EnvUtil.with_default_external): add for
  changing Encoding.default_external without warnings.

* test/ruby/envutil.rb (EnvUtil.with_default_internal): ditto.

* test/ruby/test_io_m17n.rb: use above with_default_external.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39721 b2dd03c8-39d4-4d8f-98ff-823fe69b080e