Make Mocha setup explcitly Minitest-specific

This has been possible since Mocha v1.0 and makes it clear that we want
Mocha to integrate with Minitest, not Test::Unit.

actioncable/test/test_helper.rb

@@ -4,7 +4,7 @@
 require "active_support/testing/autorun"
 
 require "puma"
-require "mocha/setup"
+require "mocha/minitest"
 require "rack/mock"
 
 begin

activerecord/test/cases/helper.rb

@@ -184,4 +184,4 @@ def in_time_zone(zone)
     end
 end
 
-require "mocha/setup" # FIXME: stop using mocha
+require "mocha/minitest" # FIXME: stop using mocha