Remove warnings on AS/test/cache/stores

Since the test are skipped when Redis | Memcache are not reacheable two
instance variables are not initialized properly but used on teardown
blocks, having warnings about undeclared variables:

- instance variable @cache not initialized (redis cache store)
- instance variable @_stores not initialized (memcache store)

Author: esparta

activesupport/test/cache/stores/mem_cache_store_test.rb

@@ -56,6 +56,8 @@ def setup
   end
 
   def after_teardown
+    return unless defined?(@_stores) # because skipped test
+
     stores, @_stores = @_stores, []
     stores.each do |store|
       # Eagerly closing Dalli connection avoid file descriptor exhaustion.

activesupport/test/cache/stores/redis_cache_store_test.rb

@@ -124,6 +124,7 @@ def build(**kwargs)
 
   class StoreTest < ActiveSupport::TestCase
     setup do
+      @cache = nil
       skip "redis server is not up" unless REDIS_UP
       @namespace = "test-#{SecureRandom.hex}"