Specify destination to 'tmp/generator_test_output'

`prepare_destination` remove destination directory.
When `tmp` is removed, `tmp/ember-rails` is also removed and
it breaks other tests in Rails 4.1 ...

Such as:
```
1) Error:
HomeControllerTest#test_page_header_should_include_link_to_asset:
ActionView::Template::Error: couldn't find file 'ember'
```
Test against Rails 4.1