Replace shutdown should-receive from Celluloid test
Replace shutdown should-receive from Celluloid test

Instead, use the mock (verb) to set an instance variable we can later read to
see if the method was at least called.

appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby