Extract invalid characters in UTF-8 to other test file

Extract invalid characters test to separated file.
Related: https://github.com/defunkt/resque/pull/784

This solves below:

```
  1) Failure:test_0015_rescues jobs with invalid UTF-8 characters(Resque) [/Users/sane/work/ruby-study/resque/test/resque_test.rb:174]:Expected [] to include "Invalid UTF-8 character in job: \"\\xFF\" from ASCII-8BIT to UTF-8".
```

Ruby2.0.0
Default source encoding is changed to UTF-8. (was US-ASCII)
https://github.com/ruby/ruby/blob/trunk/NEWS