Why this change?
The `can survive cache miss` test in `spec/requests/stylesheets_controller_spec.rb`
was failing because the file was not found on disk for the cache to be
regenerated. This is because a test in
`spec/lib/stylesheet/manager_spec.rb` was removing the entire
`tmp/stylesheet-cache` directory which is incorrect because the folder
in the test environment further segretates the stylesheet caches based
on the process of the test.
What does this change do?
1. Introduce `Stylesheet::Manager.rm_cache_folder` method for the test
environment to properly clean up the cache folder.
2. Make `Stylesheet::Manager::CACHE_PATH` a private constant since the
cache path should be obtained from the `Stylesheet::Manager.cache_fullpath` method.
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