It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
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.
Zeitwerk simplifies working with dependencies in dev and makes it easier reloading class chains.
We no longer need to use Rails "require_dependency" anywhere and instead can just use standard
Ruby patterns to require files.
This is a far reaching change and we expect some followups here.
This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
