Isolate each specs for cache store (#6450)

The cache store is explicitly used by some specs, but they were not
isolated and therefore not reliable. This fixes the issue by clearing
the cache after each specs.

config/environments/test.rb

@@ -23,6 +23,10 @@ Rails.application.configure do
   config.consider_all_requests_local       = true
   config.action_controller.perform_caching = false
 
+  # The default store, file_store is shared by processses parallely executed
+  # and should not be used.
+  config.cache_store = :memory_store
+
   # Raise exceptions instead of rendering exception templates.
   config.action_dispatch.show_exceptions = false