DEV: Fix a flaky Onceoff spec (PR #13314)

The error was:

Jobs::Onceoff can run all once off jobs without errors
     Failure/Error: j.new.execute_onceoff(nil)

     TypeError:
       can't create instance of singleton class
     # ./spec/integrity/onceoff_integrity_spec.rb:13:in `new'
     # ./spec/integrity/onceoff_integrity_spec.rb:13:in `block (3 levels) in <main>'
     # ./spec/integrity/onceoff_integrity_spec.rb:12:in `each'
     # ./spec/integrity/onceoff_integrity_spec.rb:12:in `block (2 levels) in <main>'
     # ./spec/rails_helper.rb:279:in `block (2 levels) in <top (required)>'
     # ./bundle/ruby/2.7.0/gems/webmock-3.13.0/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'

Sometimes the class found by ObjectSpace.each_object(Class) would be e.g: #<Class:#<Jobs::MigrateBadgeImageToUploads:0x00007f96f8277400>>

…instead of e.g: #<Jobs::MigrateBadgeImageToUploads:0x00007f96ffa59540>

This commit changes the #select to filter out those classes.

GitHub