Exclude stubbed classes from subclasses after teardown

[GCorbel]

This fix https://github.com/rspec/rspec-mocks/issues/1568.

Stubbed classes are excluded from parent subclasses after each spec.

The original issue :

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"

  gem "rspec", "3.12.0" # Activate the gem and version you are reporting the issue against.
  gem 'rspec-mocks', '3.12.6'
end

puts "Ruby version is: #{RUBY_VERSION}" # Ruby version is: 3.1.4

class Something; end

describe 'Test' do
  before(:each) do
    class A < Something; end
    stub_const('B', Class.new(Something))
  end

  it 'something' do
    puts Something.subclasses # => [B, A]
  end

  it 'something else' do
    puts Something.subclasses # => [B, B, A]
    # Only one occurence of B should be listed
  end
end

Now, Something.subclasses always return [B, A].