DEV: Fix test incorrectly removing stylesheet cache of other processes (#25103)

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.

spec/lib/stylesheet/manager_spec.rb

@@ -886,7 +886,7 @@ RSpec.describe Stylesheet::Manager do
 
     after do
       STDERR.unstub(:write)
-      FileUtils.rm_rf("tmp/stylesheet-cache")
+      Stylesheet::Manager.rm_cache_folder
     end
 
     it "correctly generates precompiled CSS" do