Minitest 5

Although it appears, that net-sftp depend on test-unit, it fails with recent test-unit with strange errors such as:

```
===============================================================================
Error: test_block_should_raise_not_implemented_error(Protocol::V01::TestBase)
  NoMethodError: undefined method `[]' for Net::SFTP::Protocol::V01::Attributes:Class
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/attribute.rb:125:in `[]'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/testcase.rb:683:in `run_test'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/testcase.rb:431:in `run'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/testsuite.rb:121:in `run_test'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/testsuite.rb:53:in `run'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/testsuite.rb:121:in `run_test'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/testsuite.rb:53:in `run'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/ui/testrunnermediator.rb:65:in `run_suite'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/ui/testrunnermediator.rb:44:in `block in run'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/ui/testrunnermediator.rb:100:in `with_listener'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/ui/testrunnermediator.rb:40:in `run'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/ui/testrunner.rb:40:in `start_mediator'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/ui/testrunner.rb:25:in `start'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/ui/testrunnerutilities.rb:24:in `run'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/autorunner.rb:409:in `block in run'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/autorunner.rb:465:in `change_work_directory'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/autorunner.rb:408:in `run'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit/autorunner.rb:59:in `run'
/usr/share/gems/gems/test-unit-2.5.5/lib/test/unit.rb:502:in `block (2 levels) in <top (required)>'
===============================================================================
```

It is actually designed around MiniTest 4.x test-unit wrapper shipped in Ruby. Since it seems that in future versions of Ruby, this wrapper will be removed, I'd like to suggest to migrate the test suite to plain MiniTest 5.x.