Fix unrelated errors when teardowning quanrantined specs

Since we're skipping the example in a before hook, the @_subscribers
instance variable that is normally set in the setup phase of the test by
https://github.com/rails/rails-controller-testing/blob/21014e48be124dd8d0af250b79f682519d445015/lib/rails/controller/testing/template_assertions.rb#L21
isn't set but the teardown phase of the example still happens and tries
to iterate over the array of subcribers: https://github.com/rails/rails-controller-testing/blob/21014e48be124dd8d0af250b79f682519d445015/lib/rails/controller/testing/template_assertions.rb#L54

Using an around hook allows the @_subscribers instance variable to be
set.
Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/spec_helper.rb

@@ -138,7 +138,7 @@ RSpec.configure do |config|
       .and_return(false)
   end
 
-  config.before(:example, :quarantine) do
+  config.around(:example, :quarantine) do
     # Skip tests in quarantine unless we explicitly focus on them.
     skip('In quarantine') unless config.inclusion_filter[:quarantine]
   end